Why have the leaves on my Dracaena marginata plant gone yellow?

hi Pam, Welcome to GOY, there are a few reasons why this could happen, the first is that you will find with this type of Dracaena does shed leaves in this manor which is all quite normal, as long as this is happening just from the bottom, it is nothing to worry about, all you need to do is remove them by pulling downwards off the stem. if the leaf discolouring is also on the top growth, then there are a few things that you will need to check, firstly under or over watering, the worst thing you can do to any house plant is to over water, so check that the soil is not wet, or that the plant is sitting in water, also they hate central heating, which will also be a cause for this leaf colouring, very common this time of year, in which case you just need to remove the damaged leaves, and move your plant away from radiators etc. could also be that it needs potting up, but i doubt this is the cause, as it would be a gradual leaf change not in the space of a week, but if it does look a bit pot bound and the soil surface a bit old, you can repot in spring, lastly - and i think this is very unlikely, pest or disease. - most likely one of the first three sagestions. hope this helps.