nipples getting darker before BFP??

Is this the "normal" time for this to happen if I'm pregnant? Does it happen this early? They've been getting darker since yesterday (or thats when I noticed it anyway) I'm 10 DPO today and got a glaring BFN yesterday. they are also sore and almsot always hard (my nipples usually arent hard unless im cold) but only my nipples are sore and not my actual boobs, I dont know if this is normal either