I felt my friends moms boob?

Ok so im 15 i was at my friends aunts wedding and it was near the end everybody was drunk and i was asked to get in a picture with my friend and his mom so i did she was in the middle of us and she said dont be shy and put my arm around her the person taking the picture was my friends moms friend she was drunk but my friends mom wasnt and she moved my hand down onto her boob i didnt know what just happened it turned me on though, my friend didnt notice her doing it and no one was around us at the time what does this mean? why did she do it? she has big boobs and shes attractive i got abit horny when she done it what should i do? my friends mom is a single parent aswell