Periods are getting farther apart.

I used to be like clockwork, 28 days. The last 4 months they have been getting further apart last month was 34 days, month prior 33 days, prior 30 days. I'm now at day 39 without my period. I'm 31 years old never had a bad pap. I don't know what's going on.... Oh and pee sticks say I'm not pregnant.