You don't say how long this has been going on. If it's only been a couple of days, it may just be that you have other things on your mind or stressed, etc. However, if this has nothing to do with stress etc. then you will want to go for a full physical to rule out any physical cause.
When this occurs it's known as Ejaculatory Incompetence (EI)
Here is an excellent site that explains this. Along the right hand site are additional links as well with information. read here
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