Everything has trial tennancies. It's their way of giving you permission to "live in the house" but no rights to stay in the house if they want you out for ASB or damages or rent arrears.
You may have to reapply as a joint tennancy, which means you will have to wait to be put back on the list, assessed, and then try and get another house, OR move in to this one on your own, and your Bf will have to wait till you're off of your trial tennancy to move in.
9 months is usually the basic amount for a trial, but if you have any problems during it they can make it last to about 18 months to give you chance to change or leave.
They may be leniant with you, but you applied and were assessed as a single mother to be, and now your needs aren't the same with your partner, you may even risk going down a band on the register with your partner in the picture.
I only got to move partner in cause I was a permanent tennant and had been there 3 years with no arrears or problems, but when we applied this time we had to put down as joint tennants, all his details, previous homes, income, everything.