I would do the special test Yergasons to check long head of bicep involvement, you could also check the integrity of the transverse humeral ligament with ER with palpation to see if the tendon rolls out.
If these are clear then likely it is the supra as suggested above.
After the acute phase, treated with positioning and modalities, I dive into eccentrics. Research has shown them to be effective with tendonopathies.