Same shyt happened to me this past summer when I came back from school. Started going to the gym more frequently with my boy, had me doing more cardio, longer workouts, and I think the sauana also had me shedding a lot of water weight. Didn't help that I got lazy as hell this summer and really only ate for breakfast and dinner with some bullshyt in between.
Anyways, try to avoid eating only two big meals a day; don't cram all your calories into a few meals, spread it out.
Make yourself two sandwiches a day. Have Greek yogurt with chia seeds honey and some granola. Oatmeal with sliced almonds, cinnamon and a little honey. Have a banana and another fruit.
For dinner I generally just keep it very simple, and I don't think many people would be able to do it consistently. But it works for me, its easy, and I feel well nourished with room for desert or another snack. I usually have chicken/steak/fish with either brown rice or potatoes, with some steamed veggies (carrots, asparagus, and brocoli always.)
I know a lot of guys on here probably wouldn't advise to get a mass gainer, but they definitely help me get those last calories in. I noticed they had me putting on some fat last year when I first used em (went from 150 to 170 though,) but since I started up on Up Your Mass again, I've gained like 5lbs of fairly lean gains.
Drink lots of water and get good sleep.
Also guacamole is a good snack. Don't know if its bad for you, but that shyt has a lot of calories + the tortilla chips.