My gf is on birth control, last month her cycle was 2 days, this month it was one, should we be concerned?

Probably not. Many women on hormone birth control (the pill, patch or ring) have very light menses. So long as the method is used as directed, there is no concern.
Shorter periods OK. It is common to find periods becoming shorter on birth control pills. The linning of the uterus becomes thin on these meds and then bleeding is often less and for fewer days. This is definitely a benefit of birth control pills.
Probably not. Low dose birth control pills commonly make cycles get shorter and lighter. Some women will even stop having cycles. As long as she isnt skipping or varying her time that she takes the pill, the failure rate is 1 to 2 percent. To help improve those numbers even more, use condoms as well., .