Hello and welcome to EmpowHer,
Trichomonas continues to have some mysteries because the research is not extensive. Not even the experts on the subject know more than your regular gynecologist. With that said, although it is a sexual transmitted disease, its not that cut and dry, and it does not necessarily mean your husband cheated on you.
Trichomonas can appear and/or reappear in completely monogamous relationships. This has been seen over and over in many cases. There is still no reliable test for men. Trich can also stay dormant in the body for years without causing any symptoms. So, your husband could have possibly had Trich without knowing it and may even had negative test results in the past. Keep in mind that even if he did have it, he may not have passed it to you immediately.
Secondly, Trich often accompanies gonorrhea or chlamydia. So always get tested for other STD's as well.
Another important thing to know about Trich is that not all tests are the same and differ in accuracy. The standard test used by most providers is to look at vaginal fluid under the microscope, but that misses many cases. A culture test is more successful and available in most standard laboratories. However, culture misses many cases as well. Some special labs now offer testing for trich DNA using polymerase chain reaction (PCR), which is the best by far; but it's not yet widely available.
So with that said, you may have had Trich even when your had negative results in January.
I hope this helps you see that Trich is not a black and white kind of STD. It definitely does not confirm that your husband cheated.
Below I have pasted a link to a HER article that talks more about the basics of Trich.