I just spent some time looking through 36-2903 and the PT AFI and I didn't see anything about wearing your own black pants to PT.
Note 2. Shorts will be worn with t-shirt. The lining in the PT shorts may be removed, however, do not modify the other PT items such as sleeve removal. Spandex shorts and legging (navy blue or black) may be worn under PT shorts (full length leggings may be worn during cold weather periods).
Note 18. There is no mandated maternity PT while participating in formations, unit activities, similar official events, and when mission/safety dictates.
You may want to check with your 1st Sgt and see if there's a base/squadron specific addition to the AFI before you do anything.
I was never permitted to wear civilian clothes and would have been considered out of uniform if I had. I just wore my usual pt shorts with the waistband rolled down, and bought one larger shirt that I was allowed to wear untucked. No big deal, and it fit right up until I delivered.
As far as I'm aware, black pants are always authorized with the standard PT gear anyway, so I wore those under and then just fudged with the pt shorts until they were relatively comfy.
It did look absurd, but the pt gear looks like crap on 95% of the AF population anyway, so I didn't sweat it too much.