First Period - Nursing/Pumping

I am about to be 4 months postpartum. I nurse on demand other than while I’m at work during the week. I noticed that I’ve been having heavy discharge the past week and some very slight cramping. Not sure if this means I’m going to start my first period? I also am nervous because we have had unprotected sex recently (only using withdrawal method), I would love another baby but was hoping to wait until my baby girl was at least a year old. What were some of your symptoms that you knew you were getting ready to start or thought you might be pregnant again postpartum?