In May, my professor agreed to write me letter of recommendation, but now I just graduated
Be friendly, normal, polite, and human and remember that your professors are (or at least should be) too.
If they offered to write the LORs, they want to help you out. It makes people feel good to help others especially with LORs, because you get to help someone and feel nice and authoritative.
I wouldn't even bother asking if the offer still stands. Just make sure that you tell them that you're grateful and appreciative that they made the offer, let them know if there are any deadlines that you have coming up which make the need for LORs more urgent, and you should be just fine. 
Always offer supplemental documentation if they need it.