4 months PP, milk supply suddenly dropped drastically

Erin

Im 4 months PP and I've had a great milk supply up until this week. Suddenly it seems like I'm drying up. I went back to work a month ago so I switched to mostly pumping 4-5 times a day. Things were going well until this week. When I pump, even on both sides I barely get 6 ounces, even if it's been 4-5 hours since I last pumped. I really wanted to make it to 6 months. Has this happened to anyone else? What could be going on?