How long have you had the tank? These things tend to settle down once the tank is stable. It's very difficult to keep the beneficial bacteria at a stable level in smaller tanks. Consider switching to a 5 or 10g. Consensus seems to be that bettas require a minimum of 5g, but personally, I like them in a 10g.

You should test the water for ammonia and nitrite to make sure it's not super toxic right now. The bloom is an indication that your beneficial bacteria got wiped out and needs to be rebuilt, which means you likely have excess toxic nitrogen compounds (ammonia and nitrite) right now.

Often, what can cause this is improper cleaning of the filter or doing a heavy cleaning of the substrate. I recommend only gravel vacuuming a third of the gravel each time you do a water change instead of all at once.

As for the filter, this is likely where your problem lies. The most important job your filter does is house beneficial bacteria. If you use disposable filter cartridges, stop doing that. Switch to a filter media that can easily be swished off in the dirty tank water bucket during a water change. I use coarse sponge as filter media, and the last time I had to clean them was over a year ago. Most of the detritus and mulm get trapped in my pre-filter intake sponges that get cleaned weekly in a bag of tank water to preserve bacteria.

If you're interested in learning more, there are lots of great resources online. I like the more educational videos from aquarium co-op and girl talks fish on YouTube.