Timolol can cause problems with an asthmatic.
And yes, you should be concerned.
Suggest your physician consider an alternative medication, or a protocol involving a smaller doseage.
There are no sources I've found that say or even mention medication being a factor.
The MayoClinic website is a good source of information on the causes as well as other information:
http://mayoclinic.com/health/asthma/DS00021/DSECTION=causes
Good luck at your appointment.