There is a lump under my jaw, could it be lymph node from bad tooth or is it something else?

Could be anything. Without examining you it would be difficult to say what it is. Recommend having a dr. Examine it. If you have a tooth with infection sufficient to result in lymph node enlargement, you should be seen asap by a dentist before the infection extends into bone or elsewhere. If not due to infection, may need further testing.
Many things . It can be an enlarged lymph gland, saliva gland or a myriad of things. The best bet is to apply warm moist compresses for comfort and seek care from your dentist or oral surgeon.