How do I gain the weight back?

Alright so volleyball season is pretty much over and I finally realized what happened to my body and I don't like it. I lost 10 pounds ! I'm 105 and 5'4" and I am not happy. I'm back down to a 32B bra and my ass is flatter :'( How can I get the weight back? Are there any foods I could eat or work outs I could do or just anything? Pleeeease help I hate being flat as a board.
How do I gain the weight back?
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