Would it ruin my relationship if I want to spend a weekend alone?

Different people. have different needs for alone time. Consider talking with your significant other about what a comfortable balance of together and alone time would look like for your relationship.
Not if its healthy. In a healthy relationship, it is normal to want to spend some time alone, and with other friends & family besides a partner. If someone is in a relationship where they feel afraid, ashamed or "in trouble" if they spend time away from their partner, this can be a sign of abuse or a very controlling, unhealthy relationship. A healthy relationship should make it easier to care for your own needs.
Can be good. There's no doubt that we all need some alone time, and it need not be damaging to relationships. In fact, it can be quite healthy. Difficulty sometimes arises in partnerships when individuals vary in the amount of solo time they require. In this case, open communication is vital to ensure that your partner understands your need for time for yourself, and also understands that it is not about him.
Talk with partner. Everyone needs time alone from time to time. It is important though that you communicate your needs with your partner and that they understand how you are feeling. Sometimes a weekend away can be healthy for a relationship but i would make sure both of you understand the reasons and agree so that there is no misunderstanding.