I should add that this doesn't sound exactly like syphilis, but its close enough to check out.
AJ
The only std that would cause this is syphilis.
Syphilis is only infectious at certain times, so its possible that you would have only been infected recently.
It is VERY important that you get this checked. If it is syphilis, it means your partner has either had this a long time, or was with someone else. If he/she has had this for a long time, there can be serious complications, so treatment is very important.
Aj