Wa'alaikumassalam! Stress, breastfeeding changes, and hormones can all delay your period, a few of us in the mum group also had irregular cycles after stopping BF or when stressed. Since your UPT is negative and you're feeling the usual cramps, I'd say it's likely just your body adjusting, tapi to be safe and put your mind at ease, maybe check with your doctor just to confirm everything's okay, especially since your cycle pattern shifted a bit last month jugak?