Bleeding in pregnacy is not unusual, it may or may not mean that something serious is happening. Nonetheless you should see a doctor just in case it is a miscarriage, infection and other types of serious issues. The important thing is to make sure that you are healthy.
Starsia is right, it is not uncommon and many women 'lose' or are prone to 'spotting' during pregnancy but it would be best to visit your GP soon so you can be referred to a gynaecologist.
I bled a bit during the first month, it was about week 5, it was caused by the baby embedding itself in my womb, which caused slight bleeding as part of the womb fell away. But definitly go to your hospital or doctors and get it checked
You should phone your local hospital and ask the early pregnancy unit to see you. Normally what happens is that you can get an early scan to check if all is well. It can be caused by many things though and you really must get it checked out. General advice is that if the blood is fresh red and with clots then it is more worrying than brown but I'd say again whichever you have it's important to get it checked by a professional ASAP.