I had a little bump around then too and I just found out yesterday that I am indeed having twins! I guess you'll just have to wait and see.
At 5 weeks its possible to have a hidden twin but it being your seconds, it may just be bloating and muscle memory. Though when you go in for your next doctors appointment tell them about your twin suspicion or call them and ask if they can check.
I am 7 weeks 3 days and I have a belly not huge but it there .. doctor put it down to swelling but at ultrasound found out i have double the fluid. Couldnt see another baby but we are gonna check again at 9 weeks as my hgc is very high
Since you said this was your second..you show faster with your second baby because your body already knows what to do