A 21 year old guy crushin on a 40 year old woman? - more details iin?

I'm a 21 year old single guy. I started this job as a substitute at a local library. The person who trained me happened to be a 40 something year old woman. I noticed she was quite flirtatious while showing me the ropes and I really began to enjoy her company. She was extremely nice and I found her personality to be so compelling, I just wanted to talk to her whenever I could and she seemed more than happy to reciprocate my attitude towards her. One day, she was standing very close and started making a very sexy gesture, you know, the type of body language that says " I like you ". Here's the weird part. I got an erection! Is that pathetic or what? I saw her eyes go down there too. She was shocked at first but found it to be hilarious (I could tell) and we both pretended to ignore it. I just sat down as soon as I noticed she noticed. I swear it was embarrassing but at the same time I didn't care because it's nature after all. I don't mind as long as it increased her confidence and didn't freak her out. I don't really blame myself, I blame my hormones and her for being such an attractive woman. Now she has a story she can laugh about with her friends, as I'm sure she did seeing how she is. I'm just left with feeling like a loser. Oh well, Mature woman have such power over guys like me. I guess this is probably not normal, but if any older woman wants to give insight as to what their reaction would be I'd appreciate it.