The symptoms you reported (nausea, increased breasts and light pink period 3 weeks after sexual intercourse) are not specific enough to be related only to pregnancy. The same symptoms might be present when hormonal changes due to a hormonal imbalance occur. What might have caused the hormonal imbalance is hard to say without additional examination by a gynecologist whom you are advised to visit.
It might be caused by irregularities in ovarian or pituitary gland hormonal production. The hormonal imbalance might be only temporary and might need no further treatment. On the other hand, hormonal treatment might be recommended if needed. The three negative home pregnancy test results significantly decrease the probability of pregnancy.
However, the physical examination, hormonal laboratory tests, and an ultrasound scan by a gynecologist would most precisely determine whether you are pregnant or not.
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