My classmate, Mike, told me that he saw my boyfriend leave the University (my University?) with a girl. Then he (who is he? You must state if it is your boyfriend or Mike since you mentioned two guys in the previous sentence) invited me to watch a movie with him and I accepted. On our way to the (cinema OR movie theater - not both), I ran across my boyfriend who WAS handing out leaflets (with my classmates - this does not make sense to me. Do the leaflets have your classmates on them?) to passersby (this is an outdated word, I would just say "to people walking by"). I did not know the fact (What fact? It is unclear what you are talking about) until then. Mike’s words (or just "Mike") caused misunderstanding between my boyfriend and MYSELF. I am a little bit confused about what you mean by the sentence "I ran across my boyfriend who were handing out the leaflets with my classmates to passersby." I don't understand why you include "with my classmates." Could you perhaps explain that? I hope that this helps and is relatively clear. Let me know if you have any questions about what I changed. Overall it looks very good!

Thank you! Your answers are very helpful. ^_^

That looks pretty good to me, and I'm glad I could help. I would just change "Not until then did I know that..." to "Until then I did not know that" (basically just moving where "not" is so that it is more natural). Also, I would change "the leaflets" to just "leaflets" because these leaflets are not "the" only leaflets in the world. Again, I'm glad I could help :)