According to my Davis’s Drug Guide for Nurses, the listed side effects for methylphenidate are:
hyperactivity, insomnia, restlessness, tremor, dizziness, headache, irritability, blurred vision, hypertension (high blood pressure), palpitations, tachycardia (increased heart rate), hypotension (low blood pressure), anorexia, constipation, cramps, diarrhea, dry mouth, metallic taste, nausea, vomiting, rashes, akathisia (restlessness), dyskinesia (movement disorder with diminished voluntary movements and presence of involuntary movements), fever, hypersensitivity reactions, physical dependence, psychological dependence, suppression of weight gain (in children), and tolerance (meaning your body develops a tolerance to the medication and it becomes less effective over time).
Now, if you will experience any of them depends on how your body handles the medication, but as others have mentioned, increasing the dosage will increase the side effects. Since you said you have already felt a strong heart beat, I’d recommend mentioning that to your doctor the next time you see him/her. If you really feel like your medication is not helping enough, you need to discuss that with your doctor and not adjust your medication on your own.