thanks for using healthcaremagic
I would be pleased to answer your question .
Looks like due to the force involved in throwing the ball , the muscles or ligament of the shoulder might have got strained . They were already injured and hence they have not got adequate rest and healing hasn't occurred . I suggest you should show the orthopedic doctor . They will examine the area for any tears and might ask for certain investigations if need be .
You can take Motrin
650 mg over the counter till then and take adequate rest .
Thanks and Take care