If nothing else it's a clogged pore. If you wear tight clothing or if it's in that type of area it will most likely cause some type of clogging especially if you sweat there too.
If I have a hard week at the gym I'll get them sometimes in areas I sweat a lot....
Wait it out for a few days and see if it continues to fester into a head... it will most likely go away too.
I get ingrown hairs all the time on my stomach. I haven't had a wax. maybe I should.