Hi.. Thanks for the query.. As per your complain it seems that due to injuring the inside of cheek due to toothbrush trauma there is formation of hematoma that is a harmless tumor like lump mass formed due to spillage of blood from ruptured blood vessels leading to accumulation of blood into the soft tissues forming a lump.. Nothing to worry as it will gradually educe in size and will finally resolve.. I would suggest you to start doing cool compresses over it or you can suck ice to reduce inflammation and soreness.. Take soft diet to avoid irritation.. Gradually the lump will start shrinking.. In case if the lump still does not regress, consult an Oral Physician and get evaluated.. Hope this helps.. Regards.