Swollen as in looking swollen or you think it's swollen mate? Overstretched doing what? I'm going to take a wild guess and say it's just DOMS(google this). But nevertheless do elaborate on what you mean by swollen and how you overstretched it. Highly unlikely that you tore your bicep tendon but better safe than sorry.

So what happened?

what kind of exercise ? what I can advise you to do is put some ice on it. Help the swelling go away. If it starts getting blue black, most probably it's a muscle tear.

the description he gave is extremely vague -__- . And honestly, DOMS that give swelling like a curl doesnt sound right but then again, I asked him and he insisted it's swelling. So.. yea.