Sudden weight gain?! Help please!?

Long story short... I'm a 24 year old female... approx 5'8" tall... a little over a year ago I weighed 156 (according to the Dr.); then I somehow managed to drop a TON of weight and got down to 117---for the past year or so I've maintained an average of 120 pounds (no diet at all, eating everything and anything I wanted). Now, within the past week or so, I've started gaining; I feel "squishy" and just bloated all the time & it is really starting to stress me out. The only thing I changed about my diet is I STOPPED eating out and eating fast food... I eat an apple or something similar for a snack, and cut out sodas drastically and exchanged for water, and NO, I'm definitely not preggers, and Aunt Flo came and went last week, so it's not THAT kind of bloating. Does anyone have any thoughts or recommendations so I can lose my "squishiness" again? I am so stressed and don't want to be 156 again! Please help...thank you so much!