Are you a teenager? Patellar tendonitis is very common in active adolescents. Basically the sheath around your tendon needs time to "lube up" after it grew. With time, the fascia around the tendon won't be so tightly wound and the pain will subside.
That's my guess, and I had the same thing when I was about 18 or so, but it goes away. Of course, this can occur in adulthood as well for various reasons but I suspect you just need to give it time.
I honestly don't think those straps do anything beyond a placebo effect. Your patellar tendon is attached to 2 very large bones and as a result is subject to all sorts of motion. A strap no matter how tight or strong can't feasibly hold it in place. If it gives you peace of mind that's of meaningful therapeutic benefit itself, but I highly doubt the strap serves any actual mechanical purpose.
I know someone is gonna reply and tell me their orthopedic doctor told them to wear the strap and that it works, etc. However, part of being a doctor is utilizing placebo effects to best benefit the patient. Meaning, even if your doc knows the strap does nothing mechanically, he knows it likely may make you feel more confident about your injury and what he did to fix it. Meaning, he gives you the strap, you put it on, feel more confident, return to your activities, and then attribute the success of the treatment to the strap and your doctor. In a case like this, most of the time it's not the "treatment" (aka the strap) that made you better. In a situation like this, it's usually that your body healed itself on it's own. Your doctor knows this, but he also knows that if his patient believes he solved their issue, that patient is likely to have more faith/trust in that doctor later on, which benefits the well being of both parties.
