I go for the 2 month injection, well, every 2 months. I often don't get a period because of this. Now my question is, how can this be healthy? Surely periods are there for a reason? If it's not coming out, what is happening to it?

Another thing is, I've been getting a dull pain where my left ovary is situated, and I've had an op on this ovary before for endometriosis.
Should I be worried?

Im guessing you are using the depo provera jab you really need to read up on the side effects of this and make sure you are 100% clear about them.

Birth Control Shot Depo-Provera Information (http://www.womens-health.co.uk/depo.html)

Any questions after reading this should be aimed at your practitioner.