Your tightenings and the hard feeling of something pushing up to your skin sounds exactly like BH. I had BH really early and they definitely exist early on- its just less common either to get them or maybe just feel them earlier on. My BH always make the right side of my belly harden and protrude as the uterus naturally leans more towards the right in most cases.

Re placenta I have an anterior placenta and it's usually completely fine as the placenta tends go move upwards as the uterus grows upwards.