Sounds like herpes. Go to the GUM clinic please, they can give you anti-virals that will speed the end of the outbreak if it is H, or tell you what it might be if it isn't. There is no need to be ashamed. There is also a blood test that is quite sophisticated and can tell you if the antibodies are new or not (i.e. help identify if it is a primary infection) although the NHS doesn't consider it a priority to pay for it so you may need it privately.
STD checks don't test for dormant herpes so you never really have the all clear from it.
If you had sex with DP the day before blisters came up and he isn't the carrier then very likely he might have got it if it is herpes as that is when it is at its most infectious - but likely this is a primary infection and you got it from him anyway.
Although it can lie dormant for years, or maybe you've had it for ages and it's been mistaken for thrush. The most common for primary outbreak is for it to happen in the first 10 days to a few weeks after infection so quite likely that you got it from current D.P.
It's incredibly common, not dangerous and on the rise so please don't be ashamed if it is it.
Here's an explanation about the antibodies that can be found in a blood test:
"The presence of HSV-1 or HSV-2 IgM antibodies indicates an active or recent infection. HSV-1 or HSV-2 IgG antibodies indicate a previous infection. A significant increase in HSV IgG antibodies, measured by comparing acute and convalescent samples, indicates an active or recent infection. Negative HSV antibody results mean that it is unlikely that the person has been exposed to HSV or that the body has not had time to begin producing HSV antibodies".