When i got x-rays before braces, it showed that there are no wisdom teeth but my right jaw started to be really painful for a few days when i open my mouth and chew, the gum behind the lower right 2nd molar is painful and tender. what can this be?

Some thoughts...: It's not your wisdom teeth since you don't have any. Could be food or bacteria caught behind that tooth, a periodontal issue, a bite that is slightly off, a fracture, pain due to bruxism, an aphthous ulcer, an abscess, etc, etc. Cannot tell over the internet without an exam and a radiograph. For those reasons you need to discuss this with your local dentist.

Fortunate: You’re fortunate you don't have @ wisdom tooth issue to deal with. Must common cause is gum inflammation due to compromised oral hygiene or impacted food debris, in other words the area has to be flossed daily or you need to use a WaterPik. If that doesn't work for you, see your General Dentist further investigation and treatment.