First time bleeding during sex

Here is the whole story..

I was on vacation with my boyfriend this past week. It is just me and him for the whole week so it is given that we had plenty of sex during that time. We just returned home yesterday and all was feeling pretty well. We had sex again today. At first, I noticed that i was so well lubricated and i thought it was just my normal bodily response. However, when my bf tried to change our position, he noticed blood stain on the sheet. It was an ample amount of blood (really significant amount & not just spotting) that i thought i was having my period already. We disregarded it momentarily and proceeded with our activity. After that, i hurried and washed off the stain and then rested for a while. Later i discovered that the bleeding has stopped. So my bf asked me for another round to check if the bleeding would recur but it did not. It happened about 4 hours ago and since the time it stopped up to this moment, it did not bleed again.

What was that bleeding all about?

My bf and I are not a fan of artificial birth control methods. We rely on the calendar and pull-out methods.We've been together for 9 months now and have always been engaging with the said methods for contraception. The past week that i was with him, he withdrew all the time. He never came inside me this past week since we knew that it is my ovulation period. My last period occurred on the 24th of June so counting from that time, i am just in my 3rd week. I am supposed to due this 22nd of July.

Was that already implantation bleeding or just normal tearing of my internal parts?

Please help me. I need some answers.