Oh Kathy... what a shitty situation. I'm really sorry that this has happened and I do sympathise.
I do have some sympathy for your colleague (in terms of the basic situation,not in terms of how she's handled it) insofar as (as Tamum said) she has presumably put a lot of effort into preparing the module while you've been away, and feels sad that that work has been 'wasted' if she never gets to teach on it again. However, she has gone about this in a totally unprofessional way and your HoD seems to be approaching it with a 'line of least resistance' kind of approach rather than doing what is right.
I think that you may need to be prepared to share this module with your colleague, for a while at least. However, you need to talk to you HoD and make it clear that you feel that this could have been handled better. If you'd been in my Dept (I'm HoD... for my sins!) I'd have talked all this through with you before you went on Mat. Leave. You need to protect your role as module leader, and then talk to your colleague about how best to share the work... but making it very clear that it's your module, which you are inviting her to take part in teaching. I know this may be hard if the HoD has already told her she can do it, but I'd still say that 'morally' the module is yours.
It is hard though. I've been in a similar situation to your colleague insofar as in my current post I was appointed to a job that was advertised for someone with an interest in modern/contemporary stuff. I got the job, even though I'm a medievalist. Once I was in post, I was keen to become involved in the various medieval modules (which were, as it happens, being taught largely by non-specialists, so it seemed a bit odd for me to be teaching all modern stuff while they taught the stuff that I was an expert on). Anyway, I made it clear that I was very happy to teach the modern modules (indeed, I invented three new modules in my first year and teach two of them still) but that ultimately I'd also like to teach some medieval modules too. I was prepared to be involved in quite a small way at first (maybe even just some seminars or the odd 'guest lecture' on the other chap's modules) and to gradually increase my involvement. As it happened, the teacher involved (who was a bit odd anyway) decided that he couldn't possibly 'share' and basically just picked up his toys and went to play elsewhere, leaving me the whole module. I felt really bad, because I had been prepared to compromise, but he didn't feel he could do that.
I know the situation's not the same in your case, but I guess what I'm saying is that you need to be prepared to compromise, up to a point... Academic departments are like that... things can change quite rapidly. In a way, that's why I like the job - you know you won't be doing the same thing year-in, year-out. (Eventually I took over all the medieval teaching due to various staff changes. Then a year later we made a new appointment and I've ended up sharing it again.)
Another complication in this situation is that in Arts Departments (not sure what you teach, but I think it's in Arts/Humanities, isn't it?) the Dept may not have got much/any money to cover your Mat. Leave. When I had someone on Mat. Leave last year we couldn't simply advertise for a replacement post for the term of her leave. I was allowed (oh so generous!) to apply for some extra part-time teaching to cover some of her hours, but not all. We were expected to cover it from within the Dept, just as we would if someone was on study leave. This makes it hard for the HoD not to fall into the trap of doing what your HoD has done, using existing expertise (even if it's less 'expert' expertise than that of the person who'd normally teach the module) to cover the gaps, because there may simply not have been the money to cover all the teaching needs otherwise.
Having said all that, I don't think you are being at all unreasonable to feel hurt, and I think that you need to talk to your HoD to try to sort this out. It's a shame that it's only being done now, because this could have been avoided by simply talking through all the ramifications before you ever went on leave....
Sorry if that's not very helpful. I am thinking of you and hope you manage to sort something out.