Non compete agreement after I got relieved from the company

I was working as Business development Executive for a Bangalore based startup from April-2014 to Jan-2015. During my tenure I was in interaction with their existing clients as well as the clients I acquired for them. I got my relieving letter on my last working day but they informed me that they will do the final settlement within 45 days. Yesterday they called me to do FnF settlement but asked me to sign an non-compete agreement stating I should not interact with any of their clients, directly or indirectly for a period of next 12 months. Being a Business developer I never know how the situation would pop up. In fact I have now joined one of their competitor and if I get an inbound opportunity from any of their existing clients I have to go after them. I never want to poach any of their clients directly! They are not ready to give me a solution when I asked them "what to do if one of your client approaches my current employer and my boss ask me to go after the opportunity. The only thing they are telling me is they will sue me if they get to know that my current employer acquire any of their existing clients!" MNC's will always tend to have multiple vendors and my scare is what if the MNC associated with with my old employer approach my current employer with new requirement? Please help me with the situation. Also, I was never told that I have to sign an non compete when I joined the company. After I got relieved, one month over now they are asking me to sign the agreement.