25 days late 4 negative urine pregnancy tests I am exclusively breastfeeding

I am 25 days late with 4 negative urine pregnancy tests and a negative blood test (results were <0.6ml which was ruled negative). I am exclusively breastfeeding my 5 month old. The doctor told me that I missed April’s period because I am breastfeeding and have a high level of prolactin which can stop you period. I had normal periods in January February and March and was breastfeeding. I’m confused. If it’s prolactin stopping my periods than I shouldn’t of had periods in January February or March.

Anyone else experience this?

What’s your thoughts?

What’s the next step?