What Causes Large Hematoma Formation On Abdomen Without An Injury?

I woke up one day ago with a large hematoma on the lower left abdomen. It s not cause by any trauma(I never bumped myself there). It s about 6 inches or more horizontal and 3 inches or so vertical. Any ideas to explain this ? Should this be investigated by having a MRI or other imaging? I ve had a small to medium lump to the left of this areas for at least 7 months. It s been sensitive like a mild bruise.