Not knowing you were on your period while having sex

Has this ever happened to anyone? 
I was pretty sure I wouldn't have my period until 3 days later, I wasn't spotting or heavily bleeding so me and my boyfriend decided to get frisky. We went a good 20 minutes without noticing anything, he even ate me out 😫 when we decided to go change posititions he looked at his penis and it was all covered in blood. I was so embarrassed I was shaking and crying apologizing. Luckily he was a trooper and it didn't bother him, he even joked around and said "I like it, I came, don't worry" 
It was the best sex we've had this month rather than this embarrassing inconvenience 🙈