My right knee is aching when I flex it can't bring it up what can it be?

Hard to say. While the pain on flexion can be something as simple as some degenerative changes on the back of the patella. It needs to be evaluated further for a specific diagnosis is there an effusion or swelling preventing the motion or is there some internal derangement such as a meniscal tear, loose body or ligamentous injury.
See your doctor. You could have a host of issues that could cause your knee pain, could be bursitis, tendonitis, arthritis, ligament issues, meniscal injury, sciatica etc. So you may want to see an orthopedic pain specialist that can tell you what is going and help you with the latest treatments available.
Knee cap problems. When bend your knee you increase the pressur behind the knee cap and the thigh bone, and when problem get worse debri fall from the friction and cause the knee to hear.

Related Questions

I have moderate posterior knee pain on my right knee. It's a constant dry ache and I can't stand on it for long periods of time. What is this?

Bakers cyst. There are a number of possibilities and you may want to be checked. Among the more common causes is known as a bakers cyst, which is a herniation of the posterior aspect of the knee capsule between two tendons in the back of the knee. The cyst is sometes associated with internal derangement of the knee such as a tear of the meniscus, so again, consider being checked. Thank you. Read more...