Uterus size, twins??

I am currently 12 and a half weeks pregnant and have a 14 month old who is still nursing. Recently after nursing I felt my stomach and could feel that my uterus was pretty hard and half way between my pubic bone and belly button which I’ve read shouldn’t be that high until 16-18 weeks, at my first ultrasound the doctor diagnosed me with a vanishing twin as he found one sac that appeared empty, my next doctor told me it’s too soon to make a call like that. My question is when you’re breastfeeding does it cause your uterus to go higher than normal? Or is it more likely it wasn’t a vanishing twin and just fraternal that implanted later than the first twin?