Prostate vs Bladder

I think what you are feeling is the seminal vesicles.  Here is an extract from Andr blog 'How to Milking a Bull'

Dear friends,
the comments and asks by our friend Andrew incite me to write another post, hoping that it could be useful for anybody who is interested in male milking.

Andrew’s asks are two: A. how we can recognize the seminal vesicles fluid; B. how can you tell if the person had an ejaculation or he has milked seminal fluid out from himself.
Here are my answers.

A. If you have read my previous posts, you have learnt that the male cum is properly made up by three different glandular juices, which are expelled nearly together in a normal ejaculation, but which are forced out in three different stages in a proper milking session. These juices are:
1. The Cowper glands fluid (5 %). This fluid is secreted by the Cowper or bulbourethral glands, popularly known as pre-cum. It is easily recognizable, as it is a viscous crystal-clear fluid. Its clearness characterizes it, and you cannot mistake it for other fluids. It drops out from penis when the genital system is fully excited in order to lubricate the urethra. Some men produce only a drop, others great amounts of it.
2. The prostatic fluid (30 %). It is secreted by the prostate gland. It is an opaque and dull juice, more fluid than cum. Its colour is more uniform than the colour of cum. It comes out when you massage and squeeze the prostate gland. Prostate is a very industrious gland. If you massage it, it can secrete large amounts of its juices, almost incessantly.
3. The seminal vesicles fluid (60 %). It is the biggest part of the ejaculate. It is very similar to the semen, gluey and milky, with a very changeable and uneven colour. It is difficult to massage your own seminal vesicles only with fingers, but when you are very excited, and they are very full, you can feel them as two hard bulges above your prostate gland. When you touch them, your penis starts to release nice ropes of seminal fluid (see the video below). When the male is excited, the filling of seminal vesicles is very fast. I advice to alternate prostate massage with seminal vesicles massage: while one gland is milked, the others are filling. This is a perfect trick to use a male as a bull to be milked. 

Yeah, sounds about right. So not quite the bladder, but at the very top end of the prostate. Still, if it's really as had to reach on my own as everyone says, I guess all the stretching and flexing is paying off.

Another consideration is as you bladder fills it will takes up 'spare' space in the lower region.  This may result in the prostate being pressed down moving closer to the anus increasing contact with the Aneros device. 

My bladder can fill up very quickly after a good session, I don't know why.  I don't think it's because the toy overshoots the right place but but there's no denying that even if I just went before an Aneros session I'll have to race like a pisshorse right afterward.  Maybe just peripheral stimulation?