You both could have experienced a Silent Heat too. Very scary because they can still get pregnant, but you don't know to keep them away from males. Here, I pulled this off another site:
During a silent heat, there are little, to no signs. No discharge, very little to no swelling of the vulva, etc. It's common for their first heat to be a silent heat. A few clients have come in because their bitch is "acting pregnant", only to find out via ultrasound, she is. Clients are amazed every time because they never knew she was in heat, so didn't bother to seperate her and the male. Just watch for excessive licking, odor, etc. The only other way I know you can tell for sure, is have a vaginal swab done to check for cells. Hope some of this is helpful.