Hello,
I doubt that you have hamstrings tendonitis, unless you have been completing repeptive and strenuous or high load training recently. but post op?
It is more than likely that you still have shortening of the hamstrings. If not shortening some form of myofascial issue/scarring.
Massage, although useful, would not necessarily enhance collagen alignment and fibroblast production like a good blast of Physiotherapy!
also, how is your stability? Foot position post op? Or most importantly flexibility! Hamstrings are a significant stabilizer and without training it is unknown if the 'exercise' (which one) that you are doing is exacerbateing problems.