Bikini weight

I'm 5'7, 21 years old weighing anywhere between 134-140. I swear yesterday I was 134 today I weighed at 140. Anyway, I know it's a healthy weight for my size, but my dad is always telling me what I should work out and how I should eat, like I don't look good or something. I feel like if I were 120 I'd look good in a bathing suit but no matter how much I work out and watch what I eat my weight WONT BUDGE. WHY. I've literally been 134-140 FOR YEARS.