Okay, stop with the Melafix because it will only hurt.
And even though you live in a "warmer" climate you should get a heater. When their temperature drops like that and warms back up, it's very bad for them and stresses them out. Think of it like when you sit in an air conditioned building and it feels good but then you go out into the blistering hot summer sun and how you might feel a little sick? Yeah, that's kind of how your Betta will feel :)
It's best to get a heater like this: http://www.drsfostersmith.com/produc...8&pcatid=11368
the 25 Watt one is best so you can change the temperature of his tank if you need to.
And...you should stop feeding him Peas, that's no good either unless they are really sick. What makes you think he is constipated?