involuntary wight loss

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I usually weigh around 190 lbs. 5'10". 33 yrs old. I workout hard. Very in shape. The last 5-6 months, I've lost around 15-20 lbs. But it seem's like weight I don't want to lose. My diet hasn't really changed. I'm also, feeling a little weaker. And I want to gain it back. The only thing I can think of... is, that I have been under alot of stress. Worrying too much. Thinking too much. All that kind of stuff. Is it possible this could just all be stress related? Or are there any types of health problems (off the top of your head) that could cause this. I know this probably sounds vain. But I'm really noticing loss of thickness around my shoulder's, chest, back, and other muscular areas. I'm just wondering what is wrong. I'm not doing any "cardio", in hopes to help. I even mix up my routine, to trick my muscles. I take a multi vitamin, magnesium, vitamin E, omega 3, and try to keep up my protein intake. Anything you can tell me, or suggest, would be a great help at this point. I want to gain my weight back. I'm getting very concerned.  Thank you.