Yoghurt after use by date
On this site, we will not and can not give medical advice and that includes answers about “should I be worried” or judge whether a food is spoiled (apart from blatant cases like “it’s covered in mold” or “it smells foul”). What we can and are happy to do is answer questions about food safety based on the recommendations of government sources or other qualified authorities. 

In your case, see the Food Safety Authority of Ireland’s webpage, which basically states that you yogurt is no longer safe after the use by date. 

Now, you may want to remember that food safety and a food being spoiled are not necessarily directly correlated. Food safety deals with statistics, so that the manufacturer can basically guarantee that before that date the risk of food poisoning from consuming said item is near zero (it can never be zero), even for very sensitive consumers like small children, pregnant women, the elderly or people with a weak immune system and this will typically include a bit of a safety margin as well . After that date, the “safety” can’t be guaranteed. The food may or may not contain a critical amount of microorganisms that can cause food borne illnesses.

Many consumers are willing to take a small risk by consuming foods after that date, especially for products that are not in high-risk groups like fish, shellfish or meat or that have a bit of “protection” built in like live “good” bacteria. And many will do so without negative consequences. The longer the time difference, the higher the risk, obviously, and of course any visible sign of spoilage should always be a warning sign. 

Bottom line: The basic rule of “When in doubt, throw it out!” still applies.