When was your last period? Have you been getting them or have they stopped? I ask because certain medicines could alter your periods cycle and cause it to stop, slow down, lighten up, get heavier, etc. If all the tests have been negative, I'm not sure your pregnant. However, if easing your mind is what your after, I suppose it wouldn't hurt to get one.
if you were 3-4 months pregnant the tests would probably definatly show a positive reading. all i can suggest is going to the doctor and finding out whats going on
If it's been that long you should have had a positive test by now. Most likely it's something else causing your symptoms. It could very well be the medicine you are on.