As long as your doctor and midwife think you're healthy it should be fine.
One of my ex's mum walked into hospital during labour in her UK size 4 jeans, she was tiny and barely showed a bump.
My friend, who recently had her baby, only started to show at 24 weeks and she is naturally a size 8.
As long as your doctor says you are fine then you're fine. Remember, there is a show called "I didn't know I was pregnant" so not everyone gets a bump and they still have healthy babies.
I'm sorry you're going through the "are you really pregnant" phase, not a fun one