Tubal ligation and pregnancy?

I had a tubal ligation using clamps about 4 months ago and I had a period after about 6 weeks. It has now been roughly 6 weeks since my period and was wondering if that is common? I was also wanting to know if its possible to still get pregnant when using clamps in tubal ligation? Thanks. I have three wonderful children but that is enough=) (although I would have had 4 if my husband would have said yes)