Hi... Okay, this situation happened to me for the first time yesterday. We were having sex and his penis went soft. This never happened before. He tried to blame me like I did something wrong, but I can't seem to wrap my head around it. Im 18 and he's 20. We've been together since I was 15.he said the same situation happened with his ex and he was done with her. Does this mean that he's cheating? Right now Im still living with my parents and he's living with friends.

What made the situation weirder. I was giving him a blowjob and for the first time my lips started to burn. Could this mean something serious?? Someone please help!!

Millions of penises go soft every day (out of about 2-3 billion).
It means just about anything, from tiredness, distraction, lack of interest, bad mood, enlarged prostate, all the way to ED, or erectile dysfunction, usually as men get older.
OR
It could indeed mean he's 'done with you.' Meaning - what? He no longer loves you, or he has his eye on someone else? How would we know? You might want to ask him!
Nothing proves cheating except venereal disease, with you not cheating.
Lips burning, who knows. That is not a sign of any disease I know of.
If you are at all mistrusting now, go get tested, and have a good heart to heart.

I can't say it any better than joypulv did. I suspect he was fishing for an excuse (out of embarrassment or maybe ignorance) and blamed you.

("Going soft" is not an uncommon problem, even when a guy masturbates, so I'm sure you will encounter it again during the many years of being sexually active. How you handle it is very important.)

Look at it from this perspective... have you as a women... ever wanted to orgasm and was really primed to but just wasn't able to... even though it might have been easy for you usually? (yes I know its not easy for all women, just making a point) Pretty much the same thing... lots of things come into play and almost any of them can derail it.

As was mentioned... it happens... don't try to make much of it unless it starts to happen frequently.

Does the "burning lips" indicate a concern about an STD? If so, get tested.

Otherwise, concerning the other answers - ditto.

Blaming you is a red flag that he is hiding something you need to know and if you can't talk about it, don't have sex. After 3 years of being together seems you would be further down the road of honest communications to be playing the blame game.

Burning lips?? I don't know but why take a chance. See a doctor, get tested.

I'm curious about the burning lips thing... if she finds out why I hope she comes back and tells us... I really want to know what could cause that. Out of curiosity.

Thank you all. It is the first time its happened with us. And the burning lips part I am going to check out. I appreciate the responses