am I the only one?

Hi everyone,
I am 39 years old and have been married for 12 years to a very loving and wonderful husband. When we first got married we had a very active sex life, but after real life set in about 2 years into our marriage I completly lost my libido. I have no interest in sex at all. It is not anything that my husband is doing wrong and it is not my hormones. The reason I know this is because there is a guy at work that started flirting with me, he wanted to give me a hug, I said okay, well when he hugged me my libido came back full force!
So far all we have done is kissed and I don't want to go any further, I am ate up with guilt, but now I know that it is the feeling of excitement that I am after. Someone new or someone different is all it is. I know this same thing would happen no matter who I was married too after a certain amount of time I think? I have tried my best over the years to get the excitment back but I can't.
I feel terrible for what I have done, but then again it feels really good at the time. Am I the only one this is happening too?
What should I do? I would never want to leave my husband.
But I love the feeling of excitement I am getting from this other guy.
I have explained to this guy about the excitement and we both do not want a relationship or anything we are just liking the feeling. He is single by the way.
Any advice would be appreciated.