18 pounds in 20 weeks 😱

I am 20 weeks and have gained 18 pounds and I'm not sure why and it's been getting me down. I try my best to eat healthy, rarely eat fast food. In the beginning I ate a lot of carbs because of the nausea but I'm back to my regular diet. I drink nothing but water. I can't do yoga anymore so I've been doing at least 30 min of activity everyday by walking or doing a pregnancy workout video. I just don't understand what I'm doing wrong. I'm scared of how much I'll gain in the end if I already gained this much. I'm not sure there's a solution but I just want to vent. I don't care what I look like, I just want to stay healthy through out this pregnancy. I was 150 and am now 167 168ish. I also stopped smoking cigarettes when I found out, so not sure if that is contributing. :( I'm a ftm so I just want to make sure I'm on a good track