Why am I having these UTIs?

A few weeks ago, my boyfriend and I had sex, and I got a UTI. I'd never gotten one after sex before, and we'd been doing it for three months, *all* the time. The sex a few weeks ago lasted about twenty minutes. I showered within fifteen minutes of it ending. Just like every other time. I guessed it was a one time thing. The most recent time we had sex, on Friday, lasted for two hours. After we were done, we watched TV for fifteenish minutes and had sex again. It was short, ten minutes. (we were both sore lol) I couldn't pee after, I hadn't drank anything, so I just wiped my vagina with a washcloth and hoped for the best. I didn't shower until a few hours afterwards. I noticed some discomfort and itch yesterday night, and some pain when peeing today. Is there any reason I'm getting these UTIs? They're become really annoying. I don't have a job and my mother doesn't know I'm having sex so frequently. I told her the last time I had one, and I still have Tazo and cranberry pills left from the last time, luckily, but I can't buy more. Telling her how much I'm having sex is out of the question, so I kind of need this problem to be figured out.