My dad is a speech therapist, and he would say you have soft Rs. A speech impediment is just a mental disconnect, either due to improper learning, or head trauma. Speaking is muscle memory, your diaphragm, vocal cords, and tongue all work together to make proper sounds. I don't know where you are from, but an accent can cause similar issues. You don't always soften your Rs, just typically when they are next to soft vowel sounds. I know you don't care, and I wouldn't either. There are always people out there looking to make themselves feel better about themselves.