I would call a doctor and ask why he thinks this might be happening. You might also want to consider having your doctor do a professional pregnancy test. I've been watching that show, I Didn't Know I was Pregnant, and people can get negative pregnancy results when they are actually pregnant...
Of course your grades are the most important factor when applying to college, but many people have good grades. Colleges want to see that you are a well-rounded person and will probably pick someone with a slightly lower GPA who is involved in a lot of extracurricular activities over someone...
Yes! You always need to leave a tip!!! The waitress still took your order and brought your drinks and food. I've never been a waitress, but I still feel strongly about this!