Its possible because birthcontrol is not 100% effective. You can take a test now - it will be accurate at this time.
Like degriffdog said the only way to know is to test. If you used the condom properly and it is not uncommon for you to miss periods then I wouldn't be too concerned. By 6 weeks after having sex you should an accurate result.
You need to go buy a test and just rule it out. If you are you will need to see a dr or go to planned parenthood.
I have a little cramping as with any period but just a tiny bit. And I had a clear liquid I've seen but I've had it even before that