Ash,
Hamstrings injuries are common to footballers, quite often due to an in balance (in strength or length) between the quadriceps and the hamstrings (they are apposing muscles).
The bruising you describe is blood pooling in your calf muscles. It's probably from a hamstring tear (sounds like your biceps femoris hamstring). A tear means that the blood supply to the muscle can escape and if the tear is severe enough it can seep through the fascia and gravity will pull it down the leg, hence the bruising in your calf. It's called an inter-muscular haematoma, this is ok as the pressure in the torn muscle will be less than if the blood stayed put, so helps healing. The blood will dissapate in time as the body reabsorbs it.
It is a common problem, so it quite normal. You should treat it using PRICE,
Protection, Rest, Ice it for 48-72 hours, Compress the muscle, Elevate it
It can take 21 days + to recover, you would be best to see a Physio so they can help you 'manage your recovery' - this should help prevent re-occurance. They can also see if you have in inbalance or other problem (muscle strength or length) and give you exercises to re-address the situation. Will also get you back on the pitch ASAP - and stay on ; )
Hope this help,
Goosey