Breast lump and pain

Good morning, 

I've took the plunge to ring the doctors and I have an appointment this afternoon. I feel like I have a slight lump in my left boob, little swelling but it does occasionally become painful and under my armpit and to the side of my boob.I've always had one boob larger than the other, I can't seem to feel any when my bra is on or when I'm lying down. 

My boobs are saggy and heavy after weight loss and then weight gain during my pregnancy. I have a 20 week old baby and I just can't stop crying I've obviously got the worse case scenario in my head and just feel sick to the pit of my stomach.

I'm obviously embarrassed about my boobs but thankfully have a female nurse practitioner checking me over. I just don't know what to feel, I've not told anyone about it either which probably isn't helping.