Did your doctor actually test for HPV, or were you told that the abnormality is caused by HPV (therefore they assume, most likely accurately, that you have an HPV infection)?
Pap tests only screen for cervical abnormalities. HPV is not tested unless it is specifically ordered (and a second sample is taken or the original tissue is sent back for further testing). That means that clear paps don't mean that nothing is going on, it just means nothing was detected.
As far as when you were exposed to the the strain(s) of HPV which could cause your abnormality, who knows. It could have been as long ago as your first sexual experience (not even sexual intercourse; many forms of foreplay can transmit HPV).
My husband and I have been monogamous for more than 17 years. Two years ago I was told I was positive for high risk HPV when my doctor performed my first HPV test. Thank God she did! But that's another story for another day.
HPV is a horrible "test" for fidelity. The virus can take months, years, or decades to cause detectable damage, if it causes any at all.
I hope that helps.