My wisdom teeth weren't detected on X-ray last yr. Now there are hard lumps/swelling where they should be and I have tmj/sinus issues. Could it be?

Yes. NO. Hard swellings are likely the teeth underneath the gums. Wisdom teeth not not cause TMJ or sinus problems.
What type of x ray? Sometimes you cannot see the wisdom teeth on a full mouth series, you need a panoramic x-ray in order to fully see them. I would verify with your dentist again to see if you do not have wisdom teeth, or if you just cannot see them. Tmj issues are unlikely to cause swelling or bumps in the gums.
See a dentist asap. Difficult to say without seeing the x-ray and an exam. I would seek an additional opinion to make sure.
Could be... Could be wisdom teeth. Routine cavity detection films not positioned to show wisdom tooth areas. Need x-rays of these specific areas. "bumps" could also be fro gum or bone problem. Please ask your dentist to examine and x-ray questionable areas.
Could it be what? It seems like you didn't finish asking the question. Your wisdom teeth can be painful and can definitely mimic TMJ pain.