I did a poas at just one week with DS & got a very faint bfp. We'd been trying 10 months & the only symptom I had was sore boobs. It was very very faint though.
The reason its good to wait the 2 weeks is coz hcg levels in the body get higher over the days til around 12 weeks I think, so the line will get darker.
Before AF is due there might not be enough hcg to register on the test.