Yes, any heavy network use will reduce the amount of available bandwidth for other uses. Torrent software, in particular, tends to have a noticeable effect on other network traffic for several reasons:
It is in constant use. Most other forms of traffic (web browsing, email, even online games) are only sporadically sending or receiving traffic. Usage patterns like large file downloads (and, similarly, extended playback of streaming media) tie up a consistant amount of bandwidth for a much longer amount of time.
They tend to be "greedy" with bandwidth. This is for similar reasons as #1: the torrent has a lot of data to transfer, well beyond what it can send at one time. The torrent client will try to download as much data as it can.
They are bi-directional. In my experience this is the biggest drawback to seeding torrents. Most consumer-level providers give their users asynchronous bandwidth: they allow more traffic to come downstream to the customer than they permit to return upstream. This lets them provide a "faster feeling" experience than a synchronous connection, since most end-user traffic really is downloading. But one key goal of torrents is to serve as much traffic as you pull, which screws up this model. If your upstream bandwidth is being saturated by torrents, your initial requests for connections will take a long time, and everything will feel very, very sluggish.
Most torrent clients are programmed to be very cooperative in mediating these problems. They allow you to regulate things like the number and speed of uploads, downloads, and total connections. Most even have built-in traffic profiling that supply "reasonable" values for these settings, or pre-defined profilers for various speed connections.
Try asking your roommate to configure her client for a slower connection than you have. Try limiting the number of uploads, or the upload bandwidth, and if that doesn't work, just cap the total bandwidth used by the client (say, to 1/3 of your total allocation.)