Fit lifestyle but plateau?

Hey all,
I've been trying to lose around 5lbs recently. My weight has plateaued and I'm super frustrated! No matter how hard I try, I just can't get those 5 lbs to go away...okay, I did briefly drop 4 of them literally overnight, but afterwards I felt so sick that I couldn't leave my bed for a week (and then gained the 4 back literally overnight. I was SO bummed).

Ideas? I'm 5'6" 120lbs, do quite a bit of walking (as a college student, 3-5mi a day up and down hills with a 10lb backpack) and run at least 3 miles at 6 mph at the gym every other day. I also drink about two cups of coffee per day and eat super healthy foods. FitnessPal says I net around 1,500-2,000 per day...

Thoughts?

Thanks!!

Replies