Hi, I Woke Up This Morning To Find A Bruise

Hi, i woke up this morning to find a bruise on my belly. It was quite purple and sore and about the size of a strawberry. through out the day the bruise has grown quite a bit and is now bigger than a tennis ball and is very very dark. It also has some small but dark lines going through it, that kind of look like a dark stretch mark. I have no idea how the bruise appeared as it was my day of yesterday and i didnt do anyhthing at all, so no trauma. IM concerned because its getting bigger and darker in less than 12 hours.