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If you tested negative on 6th May and had bleeding on 11 th May then in all probability you are not pregnant.The first sign to suspect pregnancy
is a missed period then nausea;vomiting;breast tenderness;giddiness and even low back pain
.See if you have any of the other symptom than low back pain .The brown discharge can be some collected blood mixed with vaginal discharge
.Less bleeding can be due to hormonal problem also and low back pain and headache due to work;tiredness
also.Please get examined by a doctor and treated accordingly.