I WANT TO GAIN WEIGHT !

like omg I’ve been trying hard to gain and maintain ! I’m an athletic female, 19 years old. 5’9 and 135 lbs. I once were 142 pounds. That was earlier this year. Idk if is it because I was more physically active (Physical Education and Track), but my body was looking right ! Now that I’ve graduated high school I and got a job, I really don’t have time to workout anymore. I’m back down to 135 lbs and I must say I’ve been stressed out the passed summer and dealt with a STD and UTI. Aside from that, does anyone have advice on me gaining weight ? I want to hit 150lbs