Ay, that faint line could def be positive, lalo na with the nipple soreness and cramps you're describing, those are actually classic early signs! A week of sore nipples and random puso pains plus the timing makes sense, kaya I'd say mag-test ulit in a day or two with first morning urine para mas clear ang line, or go straight to your OB para sa blood test na sobrang accurate at walang doubt.
A few of us mums did that faint line thing din and most turned out positive, kaya don't lose hope! The nipple tenderness especially is a dead giveaway for a lot of us. Let me know kung paano ang retest, fingers crossed for you!