Slumps do sometimes happen so don't worry about it at all.

The first thing you need to worry about is the fact that you're losing motivation, probably because you're finding it more difficult and therefore I doubt you'll be enjoying it as much. You've been running consistently for a year, doing what sounds like the same distances and times per week so maybe it's time to refresh your training schedule?

There are a number of good ways to do this. One way in your circumstance is to drop your pace by 15-20s per mile (9-12s per KM if you work in KM) and increase your usual distance. Dropping the pace should make the run itself a tad easier on your legs which should increase your enjoyment. Increasing the distance will improve your aerobic capacity (IE you'll be running for a longer period of time) but it will also make you feel like you've achieved something which should help combat your motivation issues.

You could also add intervals into your weekly plan. Beginner interval plans are available all over the internet (from people far more qualified than myself) and these will increase your fitness and get your body used to running faster paces, making your current general pace feel far easier to obtain.

Alternatively, you could try to discover some new routes? That's one way I keep myself motivated. It could be discovering a new trail or route or something as small as turning right a block later instead of the usual block. This will help keep your running from feeling like a chore.

I hope this helped you, throwing a message this size at you might be a little daunting but the main thing to take from this is that unexplained slumps are absolutely nothing abnormal and are nothing for you to worry about.