I agree with quincy. The diagnosis could not have been final after the test, and colitis is a general term that includes UC, but also infectious colitis, c-diff, and other forms of IBD. The suppositories were likely prescribed because of inflammation, possibly ulceration, seen in the rectum. I think if the sigmoid or left colon had been involved you would have gotten oral or rectal 5-asa instead of suppositories. I am surprised that prednisone was not also prescribed.
The followup appointment is where the biopsy results should be available, and where further tests to see if this is infectious colitis, or other IBD, can be ordered. A culture for c-diff should also be done.
Without the tissue results and the followup meeting with the GI, I can see how the nurse might have had limited advice. Still, since some people do not tolerate 5-asa, when you complained that things were getting worse, I think the nurse should have consulted the doc, and they might have considered substituting a steroid suppository just until your 5-asa tolerance could be sorted out. You may want to consider this in evaluating whether this GI/practice is right for you.
Do push to get your questions answered, but also realize that each case of colitis is a bit different and there are often secondary IBS symptoms involved. If things aren't clear/consistent from the beginning it may be because the picture is still developing - and not because the professionals are uncaring or incompetent. Keep reading on the Internet and posting here. Eat low-residue, and consider eating lots of anti-inflammatory foods. If you are up for heavier reading, consider http://www.slackbooks.com/ulcerativecolitis - but it may be too much information for the initial stage you are at.
Good Luck