If you are sexually active, anything is possible. I've heard of the tubes fusing it's way back together or a pregnancy implanting itself somewhere else in the body. I hate to ask because I don't want to offend, but you mentioned your last regular menstrual was 4 months ago--do you think you could be going through 'the change?'
You may as well take a pregnancy test, to rule out the possibility. At your age, it would be unusual, but if you are concerned, tests are cheap.