[Always *]

Mid- to late- month is actually kind of not specific for your period. Let me elaborate, let's say you expect your period to be in the last half of the month (over a specific week maybe?), but your cycle is actually occurring every 29-31 days, this could easily mean that your period has gradually been occurring later and later in the month (or earlier, depending how it works out) and now your period will start happening at the very end or beginning of the month instead. You know what I mean? In order to manage that, it might help to get a period tracking app such as P Tracker (it might actually be called Period Tracker, but same difference). It's available on Android and IOS devices.

Also, stress and many other things could cause your period to be late. That said, unless your under significant duress / stress, that might not be it because people don't usually miss their periods over an essay, you know what I mean (unless its a really really big-deal kind of essay) but it all depends how your body reacts to stress, if you're currently experiencing a new type of stress, that could do the trick too. Things like an illness could do it too. Hell, I've heard of people experiencing "normal" stresses and suddenly skipping / missing / having a late period even though their bodies never react like that to the normally-expected stresses (like work demands, planning a wedding, buying a house, moving to a new city, a break up, etc). An eating disorder could also do it.

If you miss two cycles or begin experiencing any other symptoms (unusual discharge, abdominal pain, etc), definitely check in with your doctor because things like cysts can also cause periods to just not happen. Often times things like cysts go away on their own, but sometimes medical intervention is needed.

Hope this helps