A Bursa is a little sac that has a small amount of slippery fluid in it, and it is found in the body where ever a tendon travels across a bone. It’s slippery quality stops the tendon from wearing or fraying.
If a tendon becomes too tight, or used too much, the sac becomes irritated, and fills up with swelling. Now instead of reducing friction it gets pinched every time the tendon pulls over it. This is bursitis.
Since the swelling is contained within the sac, it produces a clearly defined bump or lump, which is tender to touch. If you have a small to medium sized swelling that looks like a small egg, and it’s around your knee, or elbow, you may have bursitis. They also occur in shoulders and hips, though here they’re deeper and harder to locate.

Your physio can likely diagnose and treat these, or at least advise you if you need an ultrasound scan. If you think you have one, try ice and voltaren gel, and if no better seek medical advice.