Both great advice/posts above.
Yeah, im curious as well - how long did you hold the hit/s? How many? And often times conduction vaping (a surface heating up to vaporize DMT) more times than not leads to you burning the DMT to some degree. The more the DMT is burnt - the harsher it will be in taste/feeling/etc. With vaporizing freebase DMT there should be literally no feeling when the DMT enters your lungs; essentially feeling like a warm air or something similar. Not saying you burnt it, just a friendly reminder that when you're dealing with conduction style vapes - there's always some sacrifice of DMT when using them, and if not extremely careful - much of the experience can be botched.
But yeah holding the hits in is extremely important. 10-15 seconds usually ime, exhale; but when I get the hit that I KNOW is going to propel me far and feel it coming on VERY strong - I will hold that hit in for all it's worth. Reaching this specific hit - whether it's the first hit or fourth hit - once I take that it in, hold it and know "ohh man, this is gaining momentum quickly" - i'll hold that hit as long as I possibly can. Of course though, if you get a top notch vaporizer like the Glass Vapor Genie - you can easily take a dose in one full hit, leaving none behind; so in that case you can just hold that hit in as long is as needed and exclude everything I said above.

On what you said regarding the feelings you had and fighting it/etc:
For me I know what you're saying about the feeling as if the volume is turned down and things sound metallic. Often from that point, as the dosage deepens and/or as the experience gains momentum and becomes more intense - that metallic sound will regress into warbled sounds and the music/sounds break down in their structure; often drawn and and extended out into one infinite string of intersecting vibrations; and if the dosage and experience is intense/deep enough - sound/s, even music ..ALL goes out the window. Hyperspace, when it's deep enough, can completely negate any external stimulus and sounds to where eventually all you hear is Hyperspace coalescing infinitely through your entire perception and being - leaving it virtually impossible for any given stimulus to intrude on the experience.
Global summed up the sweating and 'passing out' pretty well. All i'll say is - as easy as it is to get worked up and wonder "ohh no what's going on", "I did too much, this isn't good.", "I did something wrong" or any 'fighting' type of thought; (
train your mind to stay calm through ordeals, just be a passive observer, try not to impose anything onto the experience in any way; just let it flow through you and let the experience run it's course realizing that this experience IS ephemeral and it will be over shortly). Even one little inkling of a thought can precipitate into this overwhelmingly endless array of torture/intensity - both physical feeling and mental feeling. The 'pressure' feeling that you're talking about can build and build into EXTREMELY intense levels, to where you think you're going to pop out've your own skin. Sometimes once you breakthrough - this feeling gives way into utter bliss and ecstasis; other times it can remain throughout the experience (sometimes it'll keep escalating and escalating); and for some it can be extremely uncomfortable and can spin negative connotations and experiences - given your reaction - sometimes the experience going very drab, dark, or entities/forces spinning forth to operate on some part of yourself/etc and being very rough in appearance.
And yeah, as Global said - DMT is very swift in how it comes on and leaves. It can be especially short if the hits were not held in long enough, the DMT to some degree was burnt, your metabolism is extremely efficient, or sometimes the experience can leave quicker than normal (or sometimes the experience won't come on at all or to where it's barely noticeable).
Just sit back, smile, close your eyes, let it all go for those few minutes and TRUST in what is happening - no matter how it may feel or appear. The experience can give you an endless array of worrying physical sensations/feelings. Just remember and try to keep in your mind - "this is only a few minutes and will be over shortly". As has been said on this forum many times - "DMT will give you what you need, not what you want". I find this to be absolutely accurate.
Best of vibes to you and your future experiences with DMT. <3