So, I just went clicking on every little nook and cranny on My Pitt. (If you all go to Pitt, you'll find that My Pitt is VERY complex, and you'll have to dig to find certain things).
I didn't see anything pertaining to financial aid, or even accepting an admissions offer. However, after clicking about 5000 times, there is a little student info section that shows your application history (whether undergrad or grad school), and there I can see social work listed as incomplete. But I wasn't able to click on anything! So, all I learned from this was that I am most likely using the same MyPitt account as I did in undergrad.
Maybe accepting the offer isn't active on the site yet?