Can stress cause me to have a period every two to three weeks? i have tested negative for everything. so could this be stress? i've been stressed.

Yes it can: Stress can through your cycle off. Emotional turmoil can affect your cycle. It sounds like you might need some professional help or counseling to learn to deal with the stress better. Sometimes talking to a friend or professional counselor helps. Often times adequate sleep, rest and excercise helps. Seek out some form of help for yourself and Good Luck.