Congrats OP! I’m 18 weeks along here, but a few months before I conceived with this pregnancy I had a MC at just under 6 weeks, so have experience of a successful start and an unsuccessful start. It’s totally normal to have the sensation that AF is coming in early (successful) pregnancy - it’s quite unnerving! But it’s a sign that your little one is burrowing in! When you’re worried about the cramping sensation (or even light bleeding if you get it), try to picture that - it’s setting up a secure home for the next 9 months!
The main thing I noticed with pregnancy cramps is that, unlike AF cramps, they will come and go, not be persistently in the same place for hours and hours on end. I’ve just checked my notes from around 4 weeks to confirm and I had cramping every day and it could be quite intense - sometimes it woke me up at night.
Do keep an eye on the shoulder pain, but I remember also worrying about the same thing - convincing myself I could feel twinging in my shoulder - and a doctor told me that if it was shoulder pain caused by an ectopic “You’d really know about it”. As in, it would be reeeally painful. Plus at 4 weeks, everything is only just implanting and is nowhere near big enough to be causing a rupture and internal bleeding (which causes the shoulder pain). Of course, I defer to others who have actual experience to confirm.
If you’re having nausea that’s good! My MW said nausea can be a good sign of a viable pregnancy. Though also don’t worry if you don’t get much of it yet - nausea doesn’t really kick in until around 6-7 weeks for most people I think.
By contrast, for my MC at 6 weeks, I didn’t really have any warning signs in the lead up. I had some twinging and cramping but nothing that I could read into. I didn’t have any nausea, but again that’s quite normal before 6 weeks.
There was no way I could have predicted it or done anything differently - it just wasn’t meant to be and so didn’t implant. I’m sure that won’t happen for you, but remember you can do nothing to prevent it and that your body knows best what a healthy pregnancy looks like. So I won’t say “relax” (I couldn’t) but just try to take each day and hour as it comes and be happy about the fact your body knows how to get pregnant 
.