When you say water parameters are normal, could you give the numbers please. Also, how big is your tank.
I would recommend using a liquid test, rather than strips. Liquid test is more accurate, and as you get 100s of tests from a liquid test kit, they more cost effective too. API master test kit is a good one to go with. It covers the basics and is considered reliable.
You have mentioned not having an airstone because of your filter. Does your filter provide good surface agitation for gas exchange and oxygenation?
Might you have forgotton to add water conditioner when you did your water change?
You have already conceded that the tank isnt cycled though, so testing and water changes are your go to solution. Test daily, if you see detectable ammonia or nitrite, do 25% water change. Keep this up until you are consistently seeing 0ppm ammonia and nitrite and your nitrates are rising. At this point you will be cycled and you can cut back a little on your testing/water changing routine.
However, saying all that, if your tank has been running for 3 months it probably is cycled now. Ammonia poisoning is slow acting, so possibly previous poor conditions is now catching up with your fish.
Possibly changing your filter caused a cycle crash. What exactly did you replace? Its normal to just rinse these things in tank water or dechlorinated water, if you replace them you lose beneficial bacteria. The only thing you want to be replacing are polishing pads and chemical media (activated carbon normally).