My Boyfriend shuts me out

You don't say what caused the stress. That's very important information. If he shuts you out after you two have had a fight, that's going to be even more of a problem long term. If that's how he deals with other types of stress (school, job, etc.) than it's not necessarily bad for your relationship - as long as he clearly tells you that his problems have nothing to do with you. Some people just prefer to deal with their emotions on their own and prefer to actually work on solving their problems rather than talking about it to someone who is not involved (no matter how close that person is to them). Only you know if you can be with someone like that. The main question here is whether he tells you when the problem is about your relationship. Does he?

I do not know your boyfriend, but I think I know what he fels and why h does what he does. He is likely trying to protect you.

When I'm very angry I can at times say things I will later regret. If I'm mad and my gf comes and hugs me, this will not make me less mad, but it does bring her into the "impact range" of my anger. My impression is that your partner might be similar in some way. Perhaps he shuts himselff off because he knows you cannot help him and that it will only increase the chances of him acting out or saying things to you that he will regret later. He may say: "Nothing is wrong" and he means it - he knows he simply needs time to cool down and he'll be fine one he does. But he knows that he is agitated and irritable and maybe even hostile before he cools down and thus wants to "sit this thing out".

Being angry or upset from time to tme is normal, happenes to the best of us. For some people the best way is to cool down alone, this is also normal.

The real question is: how often does it happen? Is he angry/upset all the time or most of the time? If this is starting to seem like its happening most of the time, then perhaps he is suffering from long term anxiety issues, clinical depression or something similar. In such case, this can be serious and perhaps he may need professional psychological help.

As others have said, i think he shuts down to prevent unleashing that anger on you, and later regretting. (besides i think it's better, cause maybe one day he could even hit you)
I could suggest that you can sit down with him and offer him the idea to have a "safe place" (preferably outside the house), like a coffee shop or and arcade (depends on what he is into), so that when he gets angry he could go to that place, grab a bite or a drink, think stuff out, and then come back home with a cleared mind if possible (i think this has to apply some like "time limits as well", cause i don't think it's good either that he spends all day out, unless he really needs it)
Finally, just give him some space, and try to talk everything out.