I got that for a while on my Android tablet, but now it is OK. I saw other posts that said it was erratic.
Agreed that it should not be happening, but have you tried a full restart of your phone? If it's an Android they have sometime memory leaks and should be restarted on a regular (once per 3-4 days) basis -- or so said my cellphone data provider. Good luck.
Odom (not a moderator, CSR, developer, or anything except a new player who has seen similar issues in other games).