Thrombocytopaenia (low platelet count) can happen for several different reasons, one of them being lupus. Since you have a SLE diagnosis and wrote earlier that your lupus has remained active even with treatment, it is very possible that your low platelet count is related to active lupus.
How low was it? Were you given the reading?
A slightly low platelet count (between 150 and 100) is usually not a problem unless accompanied by symptoms such as excessive bruising, uncontrolable bleeding, purpura or petechia.
Once the platelet count goes under 50 most doctors would decide to intervene and treat. If the cause is autoimmune, steroids often help bring the platelet count up. Platelet transfusions are also possible, but these only help for a short while so it is important to treat the cause of the problem at the same time.
I've had mild thrombocytopaenia many times ( between 100 and 150). It is one of the ARC criteria for SLE diagnosis and for me helped in the diagnosis process. Ive never had any serious problems assocoated with it, but have had mild symptoms such as bruising, petechia and bleeding gums.
I had low platelets for about 4 years before my lupus diagnosis. I had very lowe platelets though (they ranged between 10 and 40 for the majority of the time). I was also very bruised (an average of 50 large bruises on me at any time). Since I started treatment for my lupus my platelets miraculously increased and now they average 130 apart from the occassional drop to the 50s'.
Like Cath said, it isn't really a problem unless they are very low.