Thanks, alophysio, I appreciate the reply!

Just as with the OP, I have no idea what could've caused it. One morning it just popped up.

The one thing I can think of is that I did a large volume of running (large for me) on that previous Sunday - barefoot running clinic with drills, 5km run, then 18km run. I just took it too hard, my own stupidity. After the 18km run I was beat with lots of calf pain and some pain behind the knee. There was also quite a bit of foot pain (what I thought was blisters but turned out to just be some mild redness when I got home, I run in VFF). However I analysed myself and took note of what hurt very carefully after the run and the pain I'm getting now was definitely not a part of it at the time. All foot pain disappeared the same night, calf pain persisted for a few days and my foot didn't start hurting again until that Thursday morning when this popped up.

The worry I have is I'm scheduled to run my first HM in August - is it safe to build my distance back up slowly over the next week or so? The physio said that I can run if I can handle the pain and it shouldn't do any more damage (except for maybe the cuboid shifting again and having to go back for more manipulation), but I've since read that it can do more damage to run with this?

Edit: Oh, not sure if this could have caused it, but I just remembered: I did have to sprint for the bus in very hard boots (flat, but a very sharp heel and uncomfortable) the morning before, on Wednesday. Although I've done this before and the pain didn't come up until the following morning.