Weird acting black mollie....help

[neeker4]

Yeah so on of my two black mollies, the female I think, is acting very weird. I have a 5.5g tank with a 2" sand base and it has begun to sort of burrow or creat a crater-like hole that it sits in for long stretches of time. Does anybody know if this is normal for a mollie that I think might be pregers? Thanks for any feed back...