What is this & how to deal?

Hi I am a 17 year old girl and am hoping for some guidance/input.
I am a secure, confident person with no outstanding fears and am by no means a worrier (quite the opposite actually lol), nor do I consciously get stressed about things, - yet at the same time I periodically deal with extreme tension for no apparent reason.
I loose the ability to socialize (I technically can but find it a strain & something I really have to force myself to do) & interacting with people makes me increasingly stressed. But even if there is no outside contact, i still remain very tense and incapable of relaxing.
During this time I try everything to achieve temporary relief - baths, reading, watching tv, working out, hanging out with friends, not hanging out with friends etc. NOTHING helps. It generally lasts for about a week and then goes away as abruptly as it starts. The only thing that provides temporary relief during these times is lots of simple carb based food & chocolate. & of course the temporary relief it provides at the time turns into a long term pain in my (ever enlarging) butt! lol

I have No idea what is causing this. It's been going on for quite a while and comes and goes seemingly without reason. It does not appear to be related to my period & occurs regardless of what is going on in my life. I am typically a very healthy eater & workout daily. I'm on no medication & have no health problems.

Any ideas as to what this is & how to manage it are much appreciated! Thank you!

Hi, .C.! Do you also feel depressed when you lose the ability to socialize? It's possible to get highs and lows in your mood, and if it's really bothering you, then you should talk to a psych and tell them about this.