Red stems can be strain related, If they have the "purple" genetic the branches and stems get purple/ red. i got to look in your case if its genetic. I will answer in the Comments then. The other possability is a Phosphorus defiency. IN the Flower the Plant needs alot of Phosphorus and K( Potassium) - You can give her a good shot of any Blooming booster in that case. I let you a link to a helping site where you can read that. Its hard to say with only a few informations about your plant but with the site you will find it our and solve the Problem. Happy growing...https://www.growweedeasy.com/phosphorus-deficiency-cannabis