I'm worried what people may think about me after today...?

I was invitied to go to a friends house with a load of other friends,it was all fine but then at the last minute the girl that invited me,rings me and tells me to bring a swimming costume,I couldn't go swimming as I was on my period.It was all going alright,then her dad opend the village poole,and everyone but me wen't in,everyone was asking me why I wasn't getting in.So I just sat at the side of the poole,watching everyone dive,splash around.. I felt really awkward,as everyone was looking at me like I was disassed.Everyone was having fun,I felt really left out,and accidently tears started streaming down my face as I pretended to get my water bottle from the changing rooms.I also lied,and said I couldn't go in because I had a cold,the girls mum who invited me,atualy believed me,because I was sniffing... I just feel really down,I missed out on the fun.. :/ The rest of the day was ok...