Why are most maternity trousers over bump??

I'm looking at investing in some maternity trousers as I'm starting to show a little and my (mostly high waisted) trousers no longer fit. I'm about 90% sure my problem is bloating rather than actually developing a bump yet (14 weeks) but I'm finding anything over my tummy, like a waist band, means I start getting a low level, continuous stomach ache.

As such, I'm looking for under bump clothing that will work for both now, and when I get bigger.

The problem - most maternity clothes I'm coming across are over bump.

I'm also super concerned that that's just going to emd up making me really hot too during the summer which won't be fun either.

Where has everyone been able to find good, cheap, under bump trousers/bottoms?