13 is not great but it could be much worse.
You say it suddenly goes up to 13, but when does it do this? anytime? after/before meals?
Also have you tried to look for patterns, such as, does it tend to rise when you're stressed etc?
What insulin regime are you on?
We all have our ups and downs, but if this is happening regularly then it may be that you need to change your insulin doses, but you'd be best off speaking to your diabetes care team before you make any changes.