Not the typical advice needed for a mom

Hey guys I’m 20 years old and I’ve always had a problem with never weighing over 100 pounds! I know it’s not typical and most women would be overly happy to be my weight but I dislike it for many reasons. When I was pregnant I got up to 130 pounds and after I gave birth I immediately went back down to 112 and now 6 months postpartum I weigh 100. Clothes never fit me. My son weighs 22 pounds and I’m so tiny it’s hard for me to hold him very long and people make comments like “hes almost as big as you!”. None of my clothes fit and I have very low blood pressure and it causes me to feel light headed often. My doctor suggested I try to gain a few pounds but I really just want to weigh around 115 or 120! What are some healthy things I can do to gain weight? Are there drinks or shakes? I know I can eat “more” but seeing as how I never eat very much it’s hard I don’t want to feel bad or over full! Any advice? No rude comments and no I don’t “need a burger”