Ultrasound Low Lying Placenta

I had an ultrasound today and after the ultrasound, the doctor came in and told me everything looked good, but they want me to come back in 4 weeks so they could see everything more clearly since I’m only 18 weeks 4 days. After he talked to me, he asked his nurse if he missed anything and she point at the notes and said something about a low lying placenta, but not loud enough that I was suppose to hear. The doctor reviewed the notes and said “Nope, that’s all! Do you have any more questions?” I should’ve asked about the placenta but I could tell I wasn’t suppose to hear what the nurse said. Now I’m worried... in hind sight, he also said nonchalantly that there was some bleeding with the placenta but that typically resolves itself. Do I need to be worried?