Lately my jaw has been feeling really loose, and when i don't pay attention i go straight into an underbite. why?

Malocclusion\habit?: Either you have some form of malocclusion (bad bite), an habitual occlusion (can have different bites) or you simply are placing your jaw in an eccentric position as a habit. I would recommend that you consult with your dentist to see which one of these possibilities it might be, and whether or not you should see an orthodontist for a consultation.

See below: Why is your jaw feeling loose lately did anything happen? .. If not then you might have a bite (occulsal) problem. Go to your dentist and have he or she check your bite when you are standing up and then sitting upright in the dental chair.

Growing lower jaw: If you are still growing you might be growing into an underbite. If you are beyond growing years the repositioning of your jaw could be caused by many things. You need to get it checked out.

TMJ: May be issues with your condial joint-- i would visit a TMJ specialist.

Ask your doctor: You may be experiencing jaw hypermobility where the muscles/ligaments that stabilize the jaw are no longer functioning optimally. The underbite position may be a reflection of this hypermobile activity. It would be prudent to consult with a dentist experienced with TMJ disorders.