What Are The Dark Spots Under Breast,underarm And Knees During Pregnancy?

I am 25 years old, feet 5 6 inches, weigh 255 lbs, currently 8 months pregnant. Wiehgt gain from this pregnancy is 25 lbs. For a while now i have had a dark line under my breast, very dark ankles, very dark armpits, and sometimes my neck is dark as well. These dark patches get lighter and darker but for no apparent reason.I passed my GD test so I don't believe its a sign of diabetes and it's not dirt, what else could this be?