So on this playthrough I'm about lvl 15, and I had just completed the Covenant questline and was headed back towards Sanctuary to check on my settlements.

As I'm getting close to the drive-in I come upon a group of super mutants:

http://images.akamai.steamusercontent.com/ugc/398931014502141236/5E54EED03191D55BAC289331BD5950C1E339C228/

I retreated to a safer distance to get a better look.

If you'll notice, it looks like they're escorting what appears to be a captured settler:

(I haven't had any kidnapped settlers yet til this)

http://images.akamai.steamusercontent.com/ugc/398931014502141356/40FB4159F0D6DD4B6A383F8018CC299BDF2A15C4/

Of course battle ensues, and you'll notice the captured settler tries to stay out of the way:

http://images.akamai.steamusercontent.com/ugc/398931014502141453/35B6A9F2DD1AB61DB19C478765278CF7EA30563E/

http://images.akamai.steamusercontent.com/ugc/398931014502141542/6E2E7865528D0350CFE7263173E52F1394A5C0EC/

So after I kill the mutants she runs off and I kind of assumed she was a settler from somewhere, but sure enough she's in Sanctuary to greet me when I get back:

http://images.akamai.steamusercontent.com/ugc/398931014502142466/AA0072F73A6934C3720E5C5D997486E920A1F4B6/

So now if i try to talk to her she just gives the usual response and no dialogue options appear. I can assign her to a scavenger thingy, she'll sprint over to it then unassign herself and sprint back to that spot she was at. I've tried sending her to other settlement or to a supply line, it assigns her but she just stands there and won't move.

Any ideas?

Try talking to Preston. It might trigger the start of a radiant quest where she gets kidnapped. Then afterward you might just have to talk to her or someone else at Sanctuary, and then go back to talk to Preston to 'complete' it. That's the only thing I can think of.