It sounds perfectly healthy and reasonable to me! I know how you feel though, I want to try to loose about 7lbs a month but it's definitely not easy. Perhaps you could amp up your exercise routine? Maybe try a more brisk pace/speed walk to start? As for it being new muscle mass, I don't know how likely that would be considering your main method of exercise is walking - although it does help for sure I don't know that this would be the reason you think your weight loss is slow.
Could also be water retention, do you eat high sodium foods? Have you noticed a heavier weight around your TOM? Lots of factors can contribute to this.
All in all, I still think 5lbs/month is fantastic and on track with losing at a healthy and sustainable pace. Don't worry!
If you change nothing, nothing will change.