As a side note - having an LFS (local fish shop) that will test your water is good (and actually not standard practise) but having your own kit is oh so much better - is your lfs open at 11pm? probably not.
The best kits are the ones that have bottles of solution and test tubes - the strip type ones are notoriously inaccutrate.
The best bargain at present is the API Master range (either salt water or fresh water) you get an ammonia, nitritem nitrate an pH kit in the one box often for £20 (those individually would normally be at least double that amount) - whilst this seems 'expensive' its great to have around for peace of mind and emergencies.
Its not uncommon for a newly cycles tank to have a 'mini cycle' aka new tank syndrome - but tbh Id expect yopu to be losing many more fish and much faster than you are.
For now - dont replace lost fish for at least two weeks (preferably a month) - get some test kits of your own and watch the water daily just to be sure.
Other than that - us a waiting game - but there is always someone lurking here top help if you want/need it