Is it possible to be pregnant with no increase in bbt?

It is possible that your BBT did increase but the thermometer had a mis-read. You would need to have an increase in BBT to show that you ovulated, and you need to ovulate to get pregnant, and then the BBT needs to stay high to sustain a pregnancy. The increase doesn't have to be much (as small as .2 above the coverline can be considered, I believe).