I was talking to my DM about an encounter with Tiamut (my party and I met her while she was in human form) and I got a random idea but I don't know if it's possible. Can I as a player impregnate Tiamut while she's in her human form? Or am I grasping at messed up straws

I mean, it could happen if she allows it. Divinity does weird metaphysical things to a body, but it's not like she hasn't had children before. 

Thanks mate, this campaign just got a whole lot more interesting XD

The fact that you can even ask that question tells me you can do whatever the DM allows (assuming taimut is a misspelling of tiamet).

Tiamet being the five headed chaotic evil devil dragon goddess trapped in hell whose freedom would mean the beginning of a dragon apocalypse.

Classic bard.