If you haven't had your period yet and are still worried you can buy over the counter test kits that are sensitive enough to detect the pregnancy hormone a day before your period is due.
I won't lecture you on the failure rate of the withdrawal method however it's important to be aware that according to your dates you used this method at a time you would have been most fertile. Although it does sound like you are starting a period early. It's normal for the first bit to be as you described. Even if fertilisation occurred its too early for it to have any influence over the amount of bleeding and pain you experience.
It is likely that you are starting a period.
Take pain relief as directed on the packed and place a hot water bottle on your area of pain.
As you have irregular periods it is difficult to say just yet whether this brown and pink show will develop into a full blow period or whether you actually conceived and nature is taking its course.
See how you go on over the next few days.
When things settle down, it may be a good idea to see your doctor to find out the reason why your periods are irregular.
Irregular periods can be for a number of reasons, one of them is a hormonal imbalance, polycystic ovaries and also if you are anaemic (not enough iron in your body). For the hormone and iron your doctor can take a sample of blood and check you out for that.
For polycystic and fibroid problems you would need to have an ultrasound scan to see if you have that or not.
Best wishes.
It's only been a week since you've had sex so it's way too soon to take a test. You really should wait about two more weeks and then take one. You could have had implantation bleeding. It happens a few days after sex. Many women do not get this though. So, maybe your period is getting ready to start. Was it 3 weeks ago that it started or ended?