2003 Other Toyota Models A/C not quite right

I have a 2003 Toyota Marix with 73,000 miles on it. My A/C has been working fine until yesterday. I noticed while driving the cabin air started to smell moist (like when you can smell the rain in the air) and the A/C air was getting a little warmer so I shut off the A/C and opened the windows. A bit down the road I thought I'd turn it back on and see what happens. It was running cool again but about 10 minutes later it was warm again. After I got home my husband took a look at it and changed the cabin a/c filter. It was pretty dirty and covered with leaves. It ran cold the rest of the evening. This afternoon it ran cold for a good long while - all during my first round of errands (1 hour or so). My husband also checked the refrigerant level and it indicated that it was good. But later this afternoon when we went out it ran cold for about 10-15 minutes and went warm again. This time we left the air on pulled over and tested the refrigerant again. This time it was in the RED zone! Any thoughts on what the issue could be before we take it in. If it's something simple he can probably fix it.