I understand that there are starting to be more incidences of Lyme Disease in the UK, so, since you haven't been in the US that long - and are living, IIRC, in a wintry area, you most likely had a tick bite in the UK.
I know lots of people who have had it back in my rural Wisconsin neck of the woods, and I don't know of one of them who had TOO many problems with it with the aid of antibiotics. Some of them actually had the 'bullseye' type of red marks at the site of the bit, some of them discovered they had Lyme Disease due to feeling ill...
You may never know when you were bitten, but no need to freak as long as you're being treated for it...