Hi again - sorry I thought you said red blood - "after bed it's bright red". Sounds like something minor but still should be investigated. You can never say why now! Things happen at any time. If something has developed - either in your cervix, vagina or uterus, then the trauma (!) of sex, could lead to spotting. I don't think we can help here as it is your body that needs checking - to re-iterate - all bleeding or spotting after sex should be looked at so please go and speak to the doc to put your mind at rest!
Try not to worry and let us know the outcome!
Hurdity x