I've found this forum really useful so wanted to set out my experience in case this is of helpful to others.  This is long but I've read a lot of comments in which women are both worrried about what they've found as well as what they'd experience at the gp and hospital. 

I do a self check monthly and no change in November but at the end of last month (December) felt a large, hard mass within my breast. No other changes but it felt egg sized to me and I was terrified.

I waited until my cycle completed in case it decreased in size. It didn't and in retrospect I should have immediately booked an appt with my gp instead of waiting to book. They want to see women who have noticed a change in their breasts, go and see them and don't wait. 

When I did see her she asked for a description of what I'd noticed and then examined me - she explained what she was doing so I can repeat it at home. Start standing, examine with the pads of your three fingers and either work around like a clock hand or work inwards in sections. Do this with your other hand above your head and try it both standing and lying down. She described it the lump as large, smooth and mobile and referred me to the breast clinic.  It didn't feel mobile to me so that reinforced that until we get seen it's all (worried and often panicked) conjecture from us. 

I was extremely lucky and had a hospital appt via a cancellation within a week, I think this was partially due to family history of ovarian borderline tumour and the size of my lump.

At the hospital everyone from beginning to end was kind and clear in explaining all steps. I completed a questionnaire on my health history and family health history. I met with a registrar who asked the same questions and she then examined me. After this she said she would send me for a mammogram and a ultrasound, I'd then return to her and she'd take a biopsy. I wasn't expecting the next question so share it here as I know it can be triggering: 'are you planning children in the future'. When there's lots of emotion at play it can be useful to have a response prepared.

I then went to the mammogram and ultrasound floor and had a brief wait for a mammogram. First time having it (they take images of both breasts front and then to the side). It was a bit uncomfortable but was over in seconds. This might sound daft but I'd prefer it to getting a scale and polish at the dentist 

I then returned to the waiting room and was called for my ultrasound. Within seconds the doctor said she was viewing a 'golf ball sized' cyst that she could drain immediately.

At 6cm it was larger than what I'd seen described online as a cyst and she explained they can appear very, very quickly. It was drained in seconds- they pop a needle in it - it felt like no more than a scratch - with assurance given to me that I'd missed nothing in November. They showed the before and after to me onscreen. I then had a further mammogram in that breast which again I found reassuring and the nurse showed me the before and after image.

I then returned to see the registrar who was delighted at this outcome. No biopsy required given it was a cyst - again until the checks no-one can be certain what's going on in your breast(s). She examined me again and given the size of the cyst that had been there asked if a consultant could examine me too. Again, he was content that it was fully drained and they asked me to return next month given that it's common for cysts to refill. I was told some women might have no further instances while some women may be back every three to six months with recurrent cysts: the important thing is to return each time so each instance can be investigated. 

The gp and hospital were clear and detailed about what was coming next at each stage. I was with the gp for about 15 mins and about 3 hours at hospital. It's frightening when you first feel a lump or see a change: I immediately assumed the worst, given the size, but as you'll see on this forum a lot, most instances are benign.