Hi Missy, Had the same problem after having the boiler for 12 months. Hot water for about 5 seconds then cold to tepid.
The problem was the heat exchanger for the hot water becoming silted up stopping the hot water flow through it.
My boiler is a worcester and the engineer said that the system was probably not flushed out when the new boiler was installed.
Mine was done under the Worcester two year guarantee although the Engineer said that the exchanger had a ten year guarantee.
I dont know how much a new exchanger is but it only took him an hour to replace.