It's been four months since you bought those makeup brushes and some of them are probably floating around the bottom of your purse with loose change, chewed gum and other fantastic surprises.
Brushes harbor dirt, oil, dead skin cells and bacteria. If you can't figure out why you are breaking out, perhaps its because your brushes need a little deep cleaning! You can wipe them after regular use with a paper towel and a little brush cleaner, but once in a while you need to actually shampoo them.
Ten minutes, once or twice a month will make a difference.
Use brush cleaner or baby shampoo (way less expensive) and warm water, swirling the brushes around in a soapy cup several times.
Wipe gently and swirl on a soft cloth or paper towel. For the larger, more soaked brushes I like to tap the edge on the sink to get the excess water off.
Lay to dry overnight on a towel. In order to elevate the end of the brush to make the water flow out of the bristle side of the brush, just roll the towel on one end. Or, I like to use my window sill as I am killing two birds with one stone. Drying and drying.