To further answer your question about whether your boyfriend cheated between the time you got tested or not..... Most often, trichomoniasis is NOT tested for during a routine STI/STI screening.... So, it's VERY likely you had this previously. You can double check by asking the clinic or your healthcare provider which specific STIs/STDs you were tested for.
That being said, Trichomoniasis is generally considered to be a sexually transmitted infection (STI), but it can be picked up non-sexually, too: warm, moist environments, such as jacuzzis, public baths, toilet seats, and used towels, can sometimes host and possibly, though rarely, transmit the organism. So, this is a consideration, although, the odds are this was sexually transmitted.
In terms of cheating, men often exhibit and experience NO symptoms at all or very little symptoms which are not noticeable to them.... So, it's also possible (if you got it from him), he's had this for a little while now and didn't know - this can all depend on how long your relationship has been, what kinds of sex you are having and if you have previously used protection.
Unfortunately, there's no definitive answer I can give you aside from advising you to consider all of those factors. Hope this helps better answer your question! :-)