I had a tubal ligation 4 months ago, haven't had my period in a month and also i feel nauseous could i be pregnant?

It is possible: If you had your tubal ligation after ovulation and were having unprotected sex prior to the tubal then yes! do a pregnancy test.

Its possible: Its possible that you are pregnant but the odds are not high. If 200 women had their tubes tied, 1 might get pregnant in the next 10 years. Those aren't high odds. But if you do get pregnant, you are at high risk for a tubal pregnancy. That can be serious. It would be worth getting a pregnancy test for peace of mind if you miss a period.