Starters, usually wheat and usually ones that have been kept in a not-very-cool place for a while, begin to smell rather ‘chemical’.
What to do
This a fairly common occurrence with wheat starters, especially in summer: they start smelling of ‘pear drops’, i.e. a bit chemical. The smell is actually acetone. Under certain conditions, the lactic acid bacteria in the sourdough produce copious amounts of acetic acid which gives the familiar vinegar smell. Another couple of chemical steps and this can turn into acetone. It can be a bit alarming to sniff your sourdough and get the aroma of nail varnish remover, but it is nothing to worry about. As soon as you dilute the sourdough by refreshing it with flour and water, the smell goes.
If you want to delay the onset of the acetone effect, you can stiffen up your starter by adding flour to the pot of it that you keep in the fridge. A stiffer sour will work more slowly, but, in the long run, will produce more acetic than lactic acid.