Well, it's not unusual to be irregular in the teen years and you always have been, so there is that. However, if you had unprotected vaginal sex, you risked pregnancy. Luckily you had a negative test so most likely not pregnant. Hormones including pms can make us bloated. Pregnancy doesn't make for a fat looking neck. :>)
Periods can be late due to stress, illness, weight gain or weight loss, changes in routine or hormonal imbalance.