Are you going for routine checkups with gynae? If so, you should be able to track the growth of your baby. Small bump is ok for week 20+2 and don’t sweat about lack of symptoms. Enjoy it! Assure you that those symptoms you mentioned are NOT FUN! Most backache, leg pain occur in third trimester where your bump will balloon up really fast.
i had small bump when i was 20 weeks as well! but after i reach 22/23 it started becoming more obvious. so dont worry, slowly but surely , however , even with small bumps its okay!!