Yes, me, in spades! exactly the same of you - i was even in hysterics once because i thought my tonsils were tumours 
Firstly, no matter how much you think you might reassure yourself, Google is NOT your friend, ever! So you are banned, you hear me, banned!!
Then do please go back to your GP and speak to her, not about your breast but about your health anxiety - you can have treatment in the form of ADs (you can breast feed too) and possibly some counselling.
You did the right thing by going to the doctor about your breast but believe me, if your doctor had any reason to suspect anything sinister you would be in the breast clinic before you knew it, there would be no coming back in a months time! So rest assured - your breasts can actually feel quite lumpy too, i went for the very same reason when i was BFing and it was just normal tissue, i find my breasts can be a bit lumpy at different times of the month - and i have been to the doctors a few times now with this
but every time, it turns out to be just normal - the key is, STOP FIDDLING, because that will make the area inflammed and feel lumpy, its a vicious circle.
I don't really suffer from HA anymore, i just told myself i wasn't having it, i would literally say "no, i aint avin it" and it sort of went. Ironically i do suffer from generalised anxiety now and depression but not the HA.
It is perfectly natural though, especially when we have babies to feel scared and worriedabout our health, after all, we have children now and they depend on us, i think its very common for mums to feel anxious about things. Try to be kind to yourself - do please go to the doctor and find some time to relax - anxiety can be exhausting.