You may want to consider getting the police involved if you think your significant other has done something to allow a third party to have access to your text messages. This is "stalking" and it's illegal. People often use the word "stalk" to characterize behavior that may be annoying as something more serious. Sometimes people imagine these types of things too but if someone went to the length to do that to your phone that is real stalking. The police can investigate and they may even be able to determine what was done to your phone to allow that kind of access. I understand if you don't want to get this person in trouble though too but you should at least document what you experienced as best as you can in case you need evidence of it in the future and make it known to someone else that you trust.
You should consider performing a factory reset of your phone if you aren't going to have the incident investigated by law enforcement or someone else. If you do want to get the police involved don't do this unless they say it's ok to.
These types of behaviors are not indicators of a health relationship though and abuse can escalate into violence. The National Domestic Violence Hotline can give you advice 24/7.