Lump felt only when standing

4 days ago I found a quarter sized lump in my tight breast. I am worrying myself sick! My sister in law was just diagnosed with breast cancer and is currently going through chemo. I started my period today and the earliest I can get into any doctor is in 12 days.

I've done my research and I know at my age (28) chances are it's benign but I've also read lots of things online about younger women getting diagnosed with breast cancer and how different each case/lump was so now I don't know what to think.

My sister has been diagnosed with what she calls "fibroid tumors in her breasts" since she was 18. She had one removed and now has lots of them. I am assuming she means fibroadenomas bc I can't find anything called fibroid tumors in the breasts. She says that my lump feels nothing like hers and hers are rock hard. She says my feels glandular like swollen glands. My sister in law who has breast cancer has also felt it and she said hers was way different and rock hard which also caused pulling/puckering of the outside of her breast tissue when pressed.

The lump is on the top of my breast in the middle. I can feel a gap between the lump and the breast tissue all the way around the lump. The lump isn't hard but it isn't exactly soft. It's almost like rubbery or like a testicle as weird as that sounds. When I press on it it also slightly squishes to one side....either my fingers are rolling off of it or the lump is shifting to one side when pressure is applied. I cant really tell if it fluid filled bc it feels more solid than a water balloon. I have tried moving it around other than this and it really doesn't move unless I move my breast almost like the underside(side closest to my feet) is attached maybe?!?! The lump is smoooth with district round edges and it is wider than it is tall. It's about 1in wide by .5in tall. The weird thing is when I lie down it no longer seems like a lump and when doing a self breast exam that way I never would have thought anything if it. It almost goes into my chest wall with the other breast tissue, however when standing it is not attached to my chest wall. In order to feel for it my bra has to be off and I have to be standing. I can then feel it when sliding my hand down the front of my breast. If I slightly place my hand under my breast and lift it when feeling for it I cant feel it as prominently either.

My questions are does this sound like a fibroadenoma? Cyst? I know there is only one way to know for sure but for those of you who have had them what do you think? Has anyone had something similar that turned out to be cancer?

My breasts also get very sore mid month every month and always have. I never really did self exams until my sister in law was diagnosed with breast cancer weeks ago due to usually being so sore. I believe I would have felt this though 4 years ago when breast feeding my daughter if I had it.