Rusfox
Sounds similar to my first pregnancy story.
I did not have my period for almost two months and felt like it is just about to come (menstrualtion like cramps and nagging pains in that area of stomack). I even took the test, but it came out negative. According to my calendar method those days were supposed to be safe, so I was not careful and got pregnant what I am now very happy about.
Another month passed, still no period, the same sympthoms all the time. Took another test - this time it was positive. So I got pregnant somewhere inbetween the tests.
It was very hard to find out how long I had been pregnant that time, since I did not want any ultra sound in the pregnancy. If you don't want ultrasound, you may try to find some old good experienced midwife or obstetrician that will determine the term approximately by palpation.
Anyway, if the term is not clear in the beginning, it will become clear in the course of pregnancy. Baby's heart beats first moves, growing chart, descent into pelvis and many other signs.