Left side pain under the jaw.. is it because i clench my teeth and also forgot to wear mouthguard or is it imaybe i may be having lump?

Duration?: How long has this been occurring? There is a nog difference between sudden onset pain and something that has been going on for a while. I would suggest wearing the mouth guard for a few days to see if the if the pain subsides. Of it does not then an examination by your dentist is in order.

Lump: Have your dentist do a exam including a dental radiograph to see if it is tooth or gum related. Possibly a antibiotic may be needed if infection. After that you may rule out clenching.

Many possibilities: If you have a history of clenching/grinding and are not wearing your mouthguard consistently, the pain is very likely related. However, there are multiple other possibilities. I would suggest that you start wearing your mouthguard, stay on a soft diet, avoid chewing gum, etc. Until you can be evaluated by your dentist.

Needs to be seen: These types of situations need to be actually seen in order to give you an accurate diagnosis, tell you what's going on and what treatment is needed if any. Anything else would be a disservice to you and unlikely the right answer. Call your dentist for a complete and thorough examination.