As your girlfriend's mobile no. would, no doubt, be registered with a mobile network service provider (eg Airtel/Idea/Vodafone, get a court order and approach the company seeking copies of the CDRs (call detail records) between you & your girlfriend on said dates. It is now settled law in State vs. Navjot Sandhu (2005) Criminal Appeal 373-75 that call records are admissible in court.
The whole problem in your case is that you were in a physical relationship with her. Your only defense now would be that as you had broken up with her after a span of 5 months, clearly showing your disinterest in her, she was the one pursuing you.
I can only advise you that should such conduct on her part continue, you have no other option but to file an FIR u/Sections 154 Crpc. Unfortunately defense of stalking wouldn't apply in your case, as our Indian laws say to the contrary.
It's best that you talk to a criminal lawyer and file for a restraining order effective for a one-year period. When that ends, file another permanent RO. Whatever you do, keep a paper trail intact.