Not gaining weight


So I’m currently 20 weeks pregnant and don’t seem to be gaining weight. I’ve only gained 2.5 lbs since the start of my pregnancy. I did drop about 4-5lbs in the first trimester from being sick and have managed to gained that back but haven’t gained much more. Wondering if this is concerning? I have a doctors appointment Wednesday so I plan to ask then but just wondering if anyone else has experienced being this far along and not really gaining much weight?