I'm assuming your first flight is arriving at BOM, then departing to COK next morning. You won't have access to a lounge after arriving in BOM. I presume you'll go through immigration, then need to follow signs to domestic connections - I think. It's been a few years since I've been to BOM, and back then, the international flights were in the new terminal and domestic were in the older.

On the domestic side, there should be a lounge you can use. If it;s like international, AI isn't allowed having their own, and its a GVK lounge (airport operator - they banned airlines from building their own lounges, at least on the international side, so they run the lounge and all airlines use it). Hopefully, its post-security - many India airports, including the old domestic BOM terminal, IIRC, the AI lounge was pre-security.