Yes, I have had this. I had periods every 3 to 4 months for a year and a half when in my 20's. I was not pregnant.
I had two things going on that caused these long cycles which were a slow thyroid or what is termed hypothyroidism as well as I was producing too much estrogen (naturally through my hormonal or endocrine system).
Most likely you are not pregnant as it is pretty rare to be pregnant and not be able to detect it in a urine or blood test. I guess it could happen but again, I really doubt it. You may have a hormonal issue or a thyroid issue. Your doctor can do blood work to help determine that.
I was in line to see an endocrinologist due to my thyroid and estrogen issue but my levels evened out and went back to normal on their own before I could even get in. No idea why it happened. Those issues, in fact, would have prevented me from being able to get pregnant while they were going on. I wasn't ovulating very often and hormones weren't conducive to pregnancy. But once things resolved, I went on to have two kids and honestly, my period has been fairly regular ever since on a average length cycle of 30 days.
So, try not to worry and consider other things that can cause such long cycles as pregnancy is not the only thing. good luck