How do I fix this boo boo?

This morning the guy that I was seeing casually was talking to a gal that we both know. They have a long relationship - as friends, but very flirtatious. It does not bug me in the least. But, she and I had been online chatting, when he called in. He sent through a message that he would call me later - and did so, but when he did it was per functionary. Like he felt he had to call because he had been talking to someone else - and got caught. Frankly, in my perception it was all very high schoolish. I was annoyed, so I sent him an email that was not meant to be hing - but on re-read it really was. I told him he did not owe me anything and that if he wanted to call me great but if he didn't it was fine too. I used much harsher words. I meant what I said - but the way i said it was abrasive. How do i fix it? Any thoughts?